chan_sip: Use the server reflexive ICE candidate RTCP port as provided.

This code originally worked around an issue within res_rtp_asterisk itself.
The wrong socket was being used for the STUN check for RTCP, causing the
port to be the same as RTP. This was subsequently fixed and the RTCP port
provided for the ICE candidate is correct and does not need to be incremented.

ASTERISK-23997 #close
Reported by: Badalian Vyacheslav
Patches:
 plus1.diff submitted by Badalian Vyacheslav (license 5249)
........

Merged revisions 421909 from http://svn.asterisk.org/svn/asterisk/branches/11


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/12@421910 f38db490-d61c-443f-a65b-d21fe96a405b