V4L/DVB: Fix unlock logic at medusa_video_init

As reported by Dan Carpenter:
> This was my patch:  "cx25821: fix double unlock in medusa_video_init()"
>
> It accidentally got merged two times.  The version from the staging tree
> is not correct.  Please can you revert it:
> 7a02f549 "Staging: cx25821: fix double
> unlock in medusa_video_init()"

After reviewing the logic at the function, instead of just adding a patch to
revert the wrong one, the better is to apply a different logic: add a goto
that will always unlock and return the error.

This simplifies the code a little bit, and, instead of just return -EINVAL,
it will return the error condition reported by the called functions.

This patch is folded with another patch from Dan that added one missing
goto.

Signed-off-by: Dan Carpenter <error27@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>