Fix AST_CONTROL_PVT_CAUSE_CODE handling

When the IAX2 Who Hung Up? changes were added, they uncovered a bug in
the way AST_CONTROL_PVT_CAUSE_CODE was handled in
feature_request_and_dial().  This particular frame subtype was being
treated like more terminal control frames causing the function to be
exited prematurely.


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@369061 65c4cc65-6c06-0410-ace0-fbb531ad65f3