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>