res_rtp_asterisk: Fix ssrc change for rtcp srtp

It looks like there was a copy/paste error in ast_rtp_change_source
where if there was a rtcp srtp instance, instead of updating its
ssrc we were updating the srtp instance ssrc twice.

ASTERISK-27022 #close
Reported-by: Michael Walton

Change-Id: Ic88f3aee7227b401c58745ac265ff92c19620095