m68k: Mac DP8390 update

Fix the support for C/NET nubus ethernet cards etc. Sync up the DP8390 driver
with the latest code in the mac68k repo.
Signed-off-by: Finn Thain <fthain@telegraphics.com.au>
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>