net: cdc_ether: use dev->intf to get interface information

usbnet_cdc_update_filter was getting the interface number from the
usb_interface struct in cdc_state->control. However, cdc_ncm does
not initialize that structure in its bind function, but uses
cdc_ncm_ctx instead. Getting intf directly from struct usbnet solves
the problem.

Signed-off-by: Miguel Rodríguez Pérez <miguel@det.uvigo.gal>
Signed-off-by: Bjørn Mork <bjorn@mork.no>
Signed-off-by: David S. Miller <davem@davemloft.net>