Fix return logic for IMainlineSupplicant#addStaInterface.

The correct logic is to write to the object at
the provided _aidl_return pointer.

Bug: 365585450
Test: Manual test - bring up Mainline Supplicant in
      the framework and verify that addStaInterface
      returns a non-null object
Change-Id: Ied52ec8202735c792be2b2cf6ac68ef21e1aaf64
1 file changed