media: dvbdev: remove double-unlock

As warned by smatch:
	drivers/media/dvb-core/dvbdev.c: drivers/media/dvb-core/dvbdev.c:529 dvb_register_device() error: double unlock 'sem:&minor_rwsem'

Reported-by: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>