Fix a few fax gateway failures

Fax gateway requires knowledge of a channel's peer in a bridge. This patch
now uses the supported mechanisms to get this information.

This is acceptable for a few reasons:
* Fax gateway can only ever work in a 2-party bridge
* Fax gateway cannot work when not in a bridge
* Fax gateway cannot work without knowledge of the capabilities of both
  channels in the fax operation (it is, after all, a gateway)


git-svn-id: http://svn.asterisk.org/svn/asterisk/trunk@389799 f38db490-d61c-443f-a65b-d21fe96a405b