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+.

Change-Id: Ifcec1f077e005a2c23360dc52b2d5442be290f12
Signed-off-by: Jason Martinsen <jasonmartinsen@msn.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Git-commit: 15515aaaa69659c502003926a2067ee76176148a
Git-repo: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git


Signed-off-by: Liangliang Lu <luliang@codeaurora.org>