HID: fix lock imbalance in hidraw

Add omitted unlock_kernel() to hidraw_ioctl().
Added in 979c407e
(HID: Push down BKL into ioctl handler in hidraw).

Corresponing sparse warning:
drivers/hid/hidraw.c:267:9: warning: context imbalance in 'hidraw_ioctl': wrong count at exit
drivers/hid/hidraw.c:267:9:    context 'kernel_lock': wanted 0, got 1

Signed-off-by: Jiri Slaby <jirislaby@gmail.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>