[media] dvb_usb_v2: merge files dvb_usb_init.c and dvb_usb_dvb.c

Merge files dvb_usb_init.c and dvb_usb_dvb.c to dvb_usb_core.c.
It is still under 1000 lines of code after all the optimization
so put it one file.

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