dnet: drivers/net/dnet.c needs <linux/io.h>

On m68k:
| drivers/net/dnet.c: In function 'dnet_readw_mac':
| drivers/net/dnet.c:36: error: implicit declaration of function 'writel'
| drivers/net/dnet.c:43: error: implicit declaration of function 'readl'
| drivers/net/dnet.c: In function 'dnet_probe':
| drivers/net/dnet.c:873: error: implicit declaration of function 'ioremap'
| drivers/net/dnet.c:873: warning: assignment makes pointer from integer without a cast
| drivers/net/dnet.c:939: error: implicit declaration of function 'iounmap'

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: David S. Miller <davem@davemloft.net>