export usbnet_get_ethernet_addr from usbnet and fixed cdc_ether.c

because of using the same function get_ethernet_addr as cdc_ether.c
i export usbnet_get_ethernet_addr from usbnet and fixed cdc_ether
(suggested by Oliver Neukum).

Signed-off-by: Peter Holik <peter@holik.at>
Signed-off-by: David S. Miller <davem@davemloft.net>