res_rtp_asterisk: Set correct raddr port on RTCP srflx candidates.

RTCP ICE candidates use a base address derived from the RTP
candidate. The port on the base address was not being updated to
the RTCP port.

This change sets the base port to the RTCP port and all is well.

ASTERISK-29433

Change-Id: Ide2d2115b307bfd3c2dfbc4d187515d724519040