[media] media: correct return value in dvb_demux_poll

Data type of return value is unsigned int, but in function of dvb_demux_poll,
when the pointer of dmxdevfilter equals NULL, it will return -EINVAL, which
is invalid.

Signed-off-by: Changbing Xiong <cb.xiong@samsung.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>