cassini: Use local-mac-address prom property for Cassini MAC address

Fallback on the local-mac-address prom property if the Cassini device
does not have an address programmed in the VPD ROM. This uses the same
technique as implemented by the sungem driver.

The problem was reported by Frans van Berckel using Debian kernel 2.6.34-7
on Sun Fire V440. udev was assigning a new eth<n> device name on each reboot
because the cassini driver was using a random MAC address.

Fix tested on 2.6.34-7 and 2.6.37 Sun Fire V440. Compile tested against
2.6.36 davem/sparc-2.6.git

Reported-by: Frans van Berckel <fberckel@xs4all.nl>
Tested-by: Frans van Berckel <fberckel@xs4all.nl>
Reviewed-by: Julian Calaby <julian.calaby@gmail.com>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Richard Mortimer <richm@oldelvet.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>