[TG3]: Fix probe failure due to invalid MAC address

Some older bootcode in some devices may report 0 MAC address in
SRAM when booting up from low power state. This patch fixes the
problem by checking for a valid MAC address in SRAM and falling back
to NVRAM if necessary.

Thanks to walt <wa1ter@myrealbox.com> for reporting the problem
and helping to debug it.

Signed-off-by: Michael Chan <mchan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>