media: v4l2-ioctl.c: fix missing unlock in __video_do_ioctl()

If dev_debug was active, then the code could return without unlocking the
core mutex. Replace the return with a 'goto unlock' to ensure proper unlocking.

Fixes: 73a11062 ("v4l2-core: push taking ioctl mutex down to ioctl handler")

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>