Extract some mac addr code from SiS's driver.

Some magic may hide beyond the isa bridge. The Rx mac control
register is now set without condition.

Note: good or bad, this part of the code is quite close to sis900.c.
Signed-off-by: Francois Romieu <romieu@fr.zoreil.com>
Signed-off-by: Jeff Garzik <jgarzik@pobox.com>