chan_sip: Fix autoframing=yes.

With Asterisk 13, the structures ast_format and ast_codec changed. Because of
that, the paketization timing (framing) of the RTP channel moved away from the
formats/codecs. In the course of that change, the ptime of the callee was not
honored anymore, when the optional autoframing was enabled.

ASTERISK-25484 #close

Change-Id: Ic600ccaa125e705922f89c72212c698215d239b4