HID: winwing: Add NULL check in winwing_init_led()

devm_kasprintf() can return a NULL pointer on failure,but this
returned value in winwing_init_led() is not checked.
Add NULL check in winwing_init_led(), to handle kernel NULL
pointer dereference error.

Fixes: 266c990d ("HID: Add WinWing Orion2 throttle support")
Signed-off-by: Charles Han <hanchunchao@inspur.com>
Signed-off-by: Jiri Kosina <jkosina@suse.com>