[media] dvb_usb_v2: remove various unneeded variables

Adapter priv is not really needed, use device priv instead.
There is only driver or two using that.

Device caps are not needed. There was only "has I2C adapter"
capability defined. It is useless as we can see same just
checking existence of i2c_algo callback.

And also remove some totally not used at all variables.

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