Implement the poll callback for the dvbdev_ci file ops. The ts_poll()
function queries the DVB ring buffers for available data and space, and
reports this as appropriate. Also, set the dvb_device readers, writers
and users to proper values (one reader, one writer, two users).

This fixes the raw CI TS transport in conjunction with TVheadend's
DDCI functionality.
Signed-off-by: Daniel Scheller <d.scheller@gmx.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>