[media] saa6588: after calling CMD_CLOSE, CMD_POLL is broken

CMD_CLOSE sets data_available_for_read to 1, which is necessary to do the
wakeup call, but it is never reset to 0.

Because of this calling CMD_POLL afterwards will always return that data is
available, even if there isn't any.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>