DM9000: Add platform flag for no attached EEPROM

Allow the platform data to specify to the DM9000 driver
that there is no posibility of an attached EEPROM on the
device, so default all reads to 0xff and ignore any
write operations.

Signed-off-by: Ben Dooks <ben-linux@fluff.org>
Signed-off-by: Jeff Garzik <jeff@garzik.org>