smsc75xx: only set mac address once on bind

This patch changes when we decide what the device's MAC address
is from per ifconfig up to once when the device is connected.

Without this patch, a manually forced device MAC is overwritten
on ifconfig down/up.  Also devices that have no EEPROM are
assigned a new random address on ifconfig down/up instead of
persisting the same one.

Signed-off-by: Steve Glendinning <steve.glendinning@shawell.net>
Reported-by: Robert Cunningham <rcunningham@nsmsurveillance.com>
Cc: Bjorn Mork <bjorn@mork.no>
Cc: Dan Williams <dcbw@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>