[media] cec: poll should check if there is room in the tx queue

For POLLOUT poll only checked if the adapter was configured, not
if there was room in the transmit queue. Add that check.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>