V4L/DVB (8414): videodev/cx18: fix get_index bug and error-handling lock-ups

Fix a bug in get_index that was introduced earlier.
Also fix two error handling lock-ups in videodev and cx18 that
where found thanks to that bug.

Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>