asix: Fix asix-based cards connecting to 10/100Mbs LAN.

Add AX_MEDIUM_ENCK also when speed = 10/100Mbps. This allows my belkin
f5d5055 to work with my 100Mbps switch and with an old 10Mbps ISA card.
Without this patch, the card is recognized and the interface is brought
up fine, but no packets actually flow through the interface.

Signed-off-by: Pantelis Koukousoulas <pktoss@gmail.com>
Acked-by: David Hollis <dhollis@davehollis.com>
Signed-off-by: David S. Miller <davem@davemloft.net>