media: saa7164: fix return codes 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 at saa7164,
e. g. to return EPOLERR on errors, just like the V4L2 VB2 code.

Solves the following sparse warnings:
    drivers/media/pci/saa7164/saa7164-vbi.c:632:24: warning: incorrect type in return expression (different base types)
    drivers/media/pci/saa7164/saa7164-vbi.c:632:24:    expected restricted __poll_t
    drivers/media/pci/saa7164/saa7164-vbi.c:632:24:    got int
    drivers/media/pci/saa7164/saa7164-vbi.c:637:40: warning: incorrect type in return expression (different base types)
    drivers/media/pci/saa7164/saa7164-vbi.c:637:40:    expected restricted __poll_t
    drivers/media/pci/saa7164/saa7164-vbi.c:637:40:    got int
    drivers/media/pci/saa7164/saa7164-vbi.c:647:40: warning: incorrect type in return expression (different base types)
    drivers/media/pci/saa7164/saa7164-vbi.c:647:40:    expected restricted __poll_t
    drivers/media/pci/saa7164/saa7164-vbi.c:647:40:    got int

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