net: let KS8842 driver depend on HAS_IOMEM

Fixes this compile error on s390:

  CC      drivers/net/ks8842.o
drivers/net/ks8842.c: In function 'ks8842_select_bank':
drivers/net/ks8842.c:124: error: implicit declaration of function 'iowrite16'
drivers/net/ks8842.c: In function 'ks8842_write8':
drivers/net/ks8842.c:131: error: implicit declaration of function 'iowrite8'

Cc: Richard Rojfors <richard.rojfors.ext@mocean-labs.com>
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>