ETHOC: fix build breakage on s390

Let driver depend on HAS_IOMEM to avoid build breakage on s390:

  CC      drivers/net/ethoc.o
drivers/net/ethoc.c: In function 'ethoc_read':
drivers/net/ethoc.c:221: error: implicit declaration of function 'ioread32'
drivers/net/ethoc.c: In function 'ethoc_write':
drivers/net/ethoc.c:226: error: implicit declaration of function 'iowrite32'
drivers/net/ethoc.c: In function 'ethoc_rx':
drivers/net/ethoc.c:405: error: implicit declaration of function 'memcpy_fromio'
drivers/net/ethoc.c: In function 'ethoc_start_xmit':
drivers/net/ethoc.c:828: error: implicit declaration of function 'memcpy_toio'

Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Acked-by: Thierry Reding <thierry.reding@avionic-design.de>
Signed-off-by: David S. Miller <davem@davemloft.net>