An application can try to set brightness before all the initialization is
done, in particular before the workqueue is initialized with the call to
led_init_core().  Here's a WARNING easy to trigger:

[   36.780813] WARNING: CPU: 3 PID: 1411 at ../kernel/workqueue.c:1444 __queue_work+0x37b/0x420
[   36.780815] Modules linked in: ...
[   36.780868] CPU: 3 PID: 1411 Comm: systemd-backlig Not tainted 4.16.9-1-default #1

 openSUSE Tumbleweed (unreleased)
[   36.780868] Hardware name: Dell Inc. Precision 5510/0N8J4R, BIOS 1.6.1 12/11/2017
[   36.780870] RIP: 0010:__queue_work+0x37b/0x420
[   36.780871] RSP: 0018:ffffaced048b7d78 EFLAGS: 00010086
[   36.780873] RAX: 0000000000000000 RBX: ffffffffb3f01440 RCX: 0000000000000000
[   36.780873] RDX: ffffffffc05a90d8 RSI: 0000000000000000 RDI: ffff8eac7dce2700
[   36.780874] RBP: ffff8ea547c16400 R08: ffff8ea547800000 R09: ffff8eac7dc22700
[   36.780875] R10: 0000000000000000 R11: 0000000000000040 R12: 0000000000000003
[   36.780876] R13: 0000000000000200 R14: ffffffffc05a90d0 R15: ffff8eac7dce8600
[   36.780877] FS:  00007f871e61cf40(0000) GS:ffff8eac7dcc0000(0000) knlGS:0000000000000000
[   36.780878] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[   36.780879] CR2: 000055c91115e308 CR3: 0000000883ee0005 CR4: 00000000003606e0
[   36.780880] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
[   36.780880] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
[   36.780881] Call Trace:
[   36.780886]  queue_work_on+0x81/0x90
[   36.780889]  brightness_store+0x5d/0x90
[   36.780892]  kernfs_fop_write+0x105/0x180
[   36.780894]  __vfs_write+0x26/0x150
[   36.780897]  ? common_file_perm+0x51/0x150
[   36.780900]  ? security_file_permission+0x3c/0xb0
[   36.780901]  vfs_write+0xad/0x1a0
[   36.780903]  SyS_write+0x42/0x90
[   36.780906]  do_syscall_64+0x76/0x140
[   36.780908]  entry_SYSCALL_64_after_hwframe+0x42/0xb7
[   36.780910] RIP: 0033:0x7f871dd04c94
[   36.780910] RSP: 002b:00007ffeb3a57d38 EFLAGS: 00000246 ORIG_RAX: 0000000000000001
[   36.780912] RAX: ffffffffffffffda RBX: 000055c91115c810 RCX: 00007f871dd04c94
[   36.780912] RDX: 0000000000000001 RSI: 000055c91115c810 RDI: 0000000000000004
[   36.780913] RBP: 00007ffeb3a57e10 R08: 0000000000000003 R09: 0000000000000000
[   36.780914] R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000001
[   36.780914] R13: 000055c911158f30 R14: 000055c90f3a9a4e R15: 0000000000000004
[   36.780917] Code: 74 18 e8 49 80 00 00 48 85 c0 74 0e 48 8b 40 20 48 3b 68 08
0f 84 c2 fc ff ff 0f 0b 48 83 c4 10 5b 5d 41 5c 41 5d 41 5e 41 5f c3 <0f> 0b e9
82 fd ff ff 83 cd 02 49 8d 57 60 e9 69 fd ff ff 80 3d
[   36.780942] ---[ end trace 1fce4edad54c4017 ]---

This patch initializes and acquires the led_access mutex early in the
of_led_classdev_register function, so that any application trying to write
to sysfs to set brightness will block until initialization ends.

Signed-off-by: Luis Henriques <lhenriques@suse.com>
Acked-by: Pavel Machek <pavel@ucw.cz>
Signed-off-by: Jacek Anaszewski <jacek.anaszewski@gmail.com>