HID: remove BKL from hiddev_ioctl_usage()

The race between ioctl and disconnect is guarded by low level
hiddev device mutex (existancelock) since the commit
07903407 ("HID: hiddev cleanup -- handle all error conditions
properly"), therefore we can remove the lock_kernel() from
hiddev_ioctl_usage().

Acked-by: Oliver Neukum <oliver@neukum.org>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>