[media] rtl2832_sdr: refcount to rtl28xxu

We are consumer of DVB frontend provided by rtl28xxu module. Due to
that we must use refcount to ensure none could remove rtl28xxu when
we are alive (or when we are streaming, if more fine-grained
refcounting is wanted).

Signed-off-by: Antti Palosaari <crope@iki.fi>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>