CAN: softing driver needs IOMEM

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

Signed-Off-By: Chuck Ebbert <cebbert@redhat.com>

--- a/drivers/net/can/softing/Kconfig
+++ b/drivers/net/can/softing/Kconfig
@@ -1,6 +1,6 @@
 config CAN_SOFTING
 	tristate "Softing Gmbh CAN generic support"
-	depends on CAN_DEV
+	depends on CAN_DEV && HAS_IOMEM
 	---help---
 	  Support for CAN cards from Softing Gmbh & some cards
 	  from Vector Gmbh.