media: s3c-camif: fix return code for the polling routine

All poll handlers should return a poll flag, and not error codes. So,
instead of returning an error, do the right thing here,
e. g. to return EPOLERR on errors, just like the V4L2 VB2 code.

Solves the following sparse warning:
    drivers/media/platform/s3c-camif/camif-capture.c:604:21: warning: incorrect type in assignment (different base types)
    drivers/media/platform/s3c-camif/camif-capture.c:604:21:    expected restricted __poll_t [usertype] ret
    drivers/media/platform/s3c-camif/camif-capture.c:604:21:    got int

Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>