lan78xx: Resolve issue with changing MAC address

lan78xx: Resolve issue with changing MAC address

Current state for the lan78xx driver does not allow for changing the
MAC address of the interface, without either removing the module (if
you compiled it that way) or rebooting the machine.  If you attempt to
change the MAC address, ifconfig will show the new address, however,
the system/interface will not respond to any traffic using that
configuration.  A few short-term options to work around this are to
unload the module and reload it with the new MAC address, change the
interface to "promisc", or reboot with the correct configuration to
change the MAC.

This patch enables the ability to change the MAC address via fairly normal means...
ifdown <interface>
modify entry in /etc/network/interfaces OR a similar method
ifup <interface>
Then test via any network communication, such as ICMP requests to gateway.

My only test platform for this patch has been a raspberry pi model 3b+.

Signed-off-by: Jason Martinsen <jasonmartinsen@msn.com>
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Signed-off-by: David S. Miller <davem@davemloft.net>