This tells the modem about the WAN MAC address, which is used as source
address for vectoring error reports that are generated by the firmware.

It needs to be set early, as the MEI driver only actually writes the
value to the modem when is in reset state (i.e. the firmware has been
loaded, but connection has not started yet).

Tested-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: Jan Hoffmann <jan@3e8.eu>