pch_gbe: Do not abort probe on bad MAC

If the MAC is invalid or not implemented, do not abort the probe. Issue
a warning and prevent bringing the interface up until a MAC is set manually
(via ifconfig $IFACE hw ether $MAC).

Tested on two platforms, one with a valid MAC, the other without a MAC. The real
MAC is used if present, the interface fails to come up until the MAC is set on
the other. They successfully get an IP over DHCP and pass a simple ping and
login over ssh test.

This is meant to allow the Inforce SYS940X development board:
http://www.inforcecomputing.com/SYS940X_ECX.html
(and others suffering from a missing MAC) to work with the mainline kernel.
Without this patch, the probe will fail and the interface will not be created,
preventing the user from configuring the MAC manually.

This does not make any attempt to address a missing or invalid MAC for the
pch_phub driver.