DM9000: Use netif_msg to enable debugging options

Use the netif_msg_*() macros to enable the debugging based
on the board's msg_enable field. The output still goes via
the dev_dbg() macros, so will be tagged and output as
appropriate.

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