via-rhine: Assign random MAC address if necessary

Roger Luethi has had several reports of Rhine NICs providing
an invalid MAC address.  If so, assign a random MAC address so
the hardware can still be used.

Tested as a standalone interface, as carrier for ppp, and as a
bonding slave.

Original-patch-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>
Signed-off-by: Joe Perches <joe@perches.com>
Signed-off-by: David S. Miller <davem@davemloft.net>