net: mac8390: Allow modular build

The modular driver supports only one card, just like the built-in
driver.

Note that this limitation is a problem which affects all Nubus card
drivers, because they have to do all their own bus matching, because
Nubus still lacks the necessary driver model support.

Suggested-by: Finn Thain <fthain@telegraphics.com.au>
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: David S. Miller <davem@davemloft.net>