[media] dvb_get_firmware: updates for it913x

Changes to extract firmware for it913x devices

./get_dvb_firmware it9135
extracts
dvb-usb-it9135-01.fw
dvb-usb-it9135-02.fw

./get_dvb_firmware it9137
extracts
dvb-usb-it9137-01.fw

Signed-off-by: Malcolm Priestley <tvboxspy@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>