net/can/softing: make CAN_SOFTING_CS depend on CAN_SOFTING

The statement 'select CAN_SOFTING' may ignore the dependancies
for CAN_SOFTING while selecting CAN_SOFTING_CS, as is therefore a bad choice.

Signed-off-by: Kurt Van Dijck <kurt.van.dijck@eia.be>
Acked-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>