net/usb/mcs7830: add set_mac_address

Implement set_mac_address for mcs7830. This enables me to use it with my
cable modem.

Signed-off-by: Oliver Martin <oliver.martin@student.tuwien.ac.at>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>