net/mac89x0: Replace custom debug logging with netif_* calls

Adopt the conventional style of debug logging because it is both
shorter and more flexible.
Remove the 'version_printed' flag as the version will be printed
only once anyway (when the module loads).

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