Fix a possible NULL pointer dereference in nic_remove routine
removing the nicpf module if nic_probe fails.
The issue can be triggered with the following reproducer:

$rmmod nicvf
$rmmod nicpf

[  521.412008] Unable to handle kernel access to user memory outside uaccess routines at virtual address 0000000000000014
[  521.422777] Mem abort info:
[  521.425561]   ESR = 0x96000004
[  521.428624]   Exception class = DABT (current EL), IL = 32 bits
[  521.434535]   SET = 0, FnV = 0
[  521.437579]   EA = 0, S1PTW = 0
[  521.440730] Data abort info:
[  521.443603]   ISV = 0, ISS = 0x00000004
[  521.447431]   CM = 0, WnR = 0
[  521.450417] user pgtable: 4k pages, 48-bit VAs, pgdp = 0000000072a3da42
[  521.457022] [0000000000000014] pgd=0000000000000000
[  521.461916] Internal error: Oops: 96000004 [#1] SMP
[  521.511801] Hardware name: GIGABYTE H270-T70/MT70-HD0, BIOS T49 02/02/2018
[  521.518664] pstate: 80400005 (Nzcv daif +PAN -UAO)
[  521.523451] pc : nic_remove+0x24/0x88 [nicpf]
[  521.527808] lr : pci_device_remove+0x48/0xd8
[  521.532066] sp : ffff000013433cc0
[  521.535370] x29: ffff000013433cc0 x28: ffff810f6ac50000
[  521.540672] x27: 0000000000000000 x26: 0000000000000000
[  521.545974] x25: 0000000056000000 x24: 0000000000000015
[  521.551274] x23: ffff8007ff89a110 x22: ffff000001667070
[  521.556576] x21: ffff8007ffb170b0 x20: ffff8007ffb17000
[  521.561877] x19: 0000000000000000 x18: 0000000000000025
[  521.567178] x17: 0000000000000000 x16: 000000000000010ffc33ff98 x8 : 0000000000000000
[  521.593683] x7 : 0000000000000000 x6 : 0000000000000001
[  521.598983] x5 : 0000000000000002 x4 : 0000000000000003
[  521.604284] x3 : ffff8007ffb17184 x2 : ffff8007ffb17184
[  521.609585] x1 : ffff000001662118 x0 : ffff000008557be0
[  521.614887] Process rmmod (pid: 1897, stack limit = 0x00000000859535c3)
[  521.621490] Call trace:
[  521.623928]  nic_remove+0x24/0x88 [nicpf]
[  521.627927]  pci_device_remove+0x48/0xd8
[  521.631847]  device_release_driver_internal+0x1b0/0x248
[  521.637062]  driver_detach+0x50/0xc0
[  521.640628]  bus_remove_driver+0x60/0x100
[  521.644627]  driver_unregister+0x34/0x60
[  521.648538]  pci_unregister_driver+0x24/0xd8
[  521.652798]  nic_cleanup_module+0x14/0x111c [nicpf]
[  521.657672]  __arm64_sys_delete_module+0x150/0x218
[  521.662460]  el0_svc_handler+0x94/0x110
[  521.666287]  el0_svc+0x8/0xc
[  521.669160] Code: aa1e03e0 9102c295 d503201f f9404eb3 (b9401660)

Fixes: 4863dea3

 ("net: Adding support for Cavium ThunderX network controller")
Signed-off-by: Lorenzo Bianconi <lorenzo.bianconi@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>