Light refactoring of the code in runtime_svc.c file.

 - Declare validate_rt_svc_desc()'s argument as const.

 - Remove 'goto' path in runtime_svc_init(). It was used in one
   place only.

 - Improve code readability by declaring a local variable holding the
   service pointer.

Change-Id: I3b15c5adb9f37b786b5b993a9be70ea9dd017a83