[media] dvb_usb_v2: rename module dvb_usbv2 => dvb_usb_v2

I think it is better name. At that phase renaming is quite painless
as module is not yet merged to mainline Kernel.

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