media: cobalt: remove redundant zero check on retval

The error handling paths all end up with retval being non-zero,
so the check for retval being zero is always false and hence
is redundant. Remove it.

Detected by CoverityScan CID#1309479 ("Logically dead code")

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Hans Verkuil <hansverk@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>