CRM-14034 - Fix soft credit form rule to handle editing contributions using a...

CRM-14034 - Fix soft credit form rule to handle editing contributions using a price set AND with linked soft credit(s).

----------------------------------------
* CRM-14034: Editing a contribution from a PCP gets an error saying "soft credit amount cannot be more than the total amount."
  http://issues.civicrm.org/jira/browse/CRM-14034