net: usb: ax88179_178a: write mac to hardware in get_mac_addr

When the MAC address is supplied via device tree or a random
MAC is generated it has to be written to the asix chip in
order to receive any data.

Previously in 9fb137ae ("net: usb: ax88179_178a: allow
optionally getting mac address from device tree") this line was
omitted because it seemed to work perfectly fine without it.

But it was simply not detected because the chip keeps the mac
stored even beyond a reset and it was tested on a hardware
with an integrated UPS where the asix chip was permanently
powered on even throughout power cycles.

Fixes: 9fb137ae ("net: usb: ax88179_178a: allow optionally getting mac address from device tree")
Signed-off-by: Peter Fink <pfink@christ-es.de>
Signed-off-by: David S. Miller <davem@davemloft.net>