[media] dvb_usb_core: fix sparse warning

drivers/media/usb/dvb-usb-v2/dvb_usb_core.c:24:5: warning: symbol 'dvb_usbv2_disable_rc_polling' was not declared. Should it be static?

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>