Updated smsc95xx driver to check for a valid MAC address in eeprom before...

Updated smsc95xx driver to check for a valid MAC address in eeprom before using smsc95xx.macaddr parameter passed on command line.

The built-in RPi adaptor will still get a MAC address based on the parameter passed on the command line as the RPi hardware does not have an eeprom,
however usb->ethernet adaptors using the same driver should have an eeprom with MAC address as part of their hardware and therefore will use this
meaning they don't end up with the same MAC address as the built-in RPi adaptor.