net: cdc_ether: export usbnet_cdc_update_filter

This makes the function available to other drivers, like cdc_ncm.

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