[media] dvb_usb_v2: remove num_adapters_initialized variable

We can live easily without it so remove it, make code and binary
few bytes smaller.

>From struct dvb_usb_device variable int num_adapters_initialized.

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