Fix for sending multiple '180 Ringing' messages, REF 15988

It occured that changing the condition for AST_CONTROL_RINGING
in a fix for REF 15850 introduced a degrade so the '180 RINGING'
SIP event is sent many times for the incoming call processed to DUT.
Restore the previous condition and fix the issue of early-media with
reset the flag 'pa->sentringing' in other place makes the issue of
early-media REF 15850 is fixed and there is no longer multiple
'180 Ringing' SIP messages sent to the Caller.