[media] dvb_usb_v2: make local function dvb_usb_v2_generic_io() static

dvb_usb_v2_generic_io() was not declared. It should be static.

Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Acked-by: Antti Palosaari <crope@iki.fi>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>