media: cx18: remove redundant zero check on retval

The check for a zero retval is redundant as all paths that lead to
this point have set retval to an error return value that is non-zero.
Remove the redundant check.

Detected by CoverityScan, CID#102589 ("Logically dead code")

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>