net/macsonic: Convert to nubus_driver

This resolves an old issue preventing any NuBus SONIC NICs from
working in a Mac with an on-board SONIC device.

Tested-by: Stan Johnson <userm57@yahoo.com>
Signed-off-by: Finn Thain <fthain@telegraphics.com.au>
Signed-off-by: David S. Miller <davem@davemloft.net>