net: smc911x: Automatically Update ethaddr with MAC

The ethernet controller can read the MAC from EEPROM and display it,
but if ethaddr is not set, the ethernet is still unavailable.

This patch checks will automatically set the MAC address if it has
not already been set.

Signed-off-by: Adam Ford <aford173@gmail.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>