[PATCH] bcm43xx: check for valid MAC address in SPROM

Check for valid MAC address in SPROM fields instead of relying on
PHY type while setting the MAC address in the networking subsystem,
as some devices have multiple PHYs.

Signed-off-by: Stefano Brivio <stefano.brivio@polimi.it>
Signed-off-by: John W. Linville <linville@tuxdriver.com>