[media] dvb_usb_v2: init I2C and USB mutex earlier

Those must be initialized earlier as we now pass
(struct dvb_usb_device *) to the firmware download
callbacks too.

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