CAN: softing driver depends on IOMEM

Without this dependency the softing driver will be buildable on s390,
where it fails.

Signed-Off-By: Chuck Ebbert <cebbert@redhat.com>
Acked-by: Kurt Van Dijck <kurt.van.dijck@eia.be>

Signed-off-by: David S. Miller <davem@davemloft.net>