V4L/DVB (4090): Fix cinergyt2_poll() to allow non-blocking IO on frontend

cinergyt2_poll()" shouldn't return (POLLIN | POLLRDNORM | POLLPRI) when
there are no pending events. User space programs that do non-bocking IO
using "select()" and/or "poll()" would otherwise produce high system load.
Acked-by: Andreas Oberritter <obi@linuxtv.org>

Signed-off-by: Axel Dyks <xl@xlsigned.net>
Acked-by: Andreas Oberritter <obi@linuxtv.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>