au1000_eth: fix invalid address accessing the MAC enable register

"aup->enable" holds already the address pointing to the MAC enable
register. The bug was introduced by commit d0e7cb:

"au1000-eth: remove volatiles, switch to I/O accessors".

CC: Florian Fainelli <florian@openwrt.org>
Signed-off-by: Wolfgang Grandegger <wg@denx.de>
Acked-by: Florian Fainelli <florian@openwrt.org>
Signed-off-by: David S. Miller <davem@davemloft.net>