V4L/DVB (9495): cx88-blackbird: bugfix: cx88-blackbird-poll-fix

Starts encoder not only on a read call but also on a poll command.

Signed-off-by: Frederic CAND <frederic.cand@anevia.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>