Pass the correct value to ast_timer_set_rate() for IAX2 trunking.

Pass the correct value to ast_timer_set_rate() for IAX2 trunking.

IAX2 uses the trunkfreq variable to determine how often to send trunk packets, but
this value is in milliseconds while ast_timer_set_rate() expects the rate argument
to be ticks per second.  So we divide 1000 by trunkfreq and pass that in instead.

With a default of 20ms, this change makes IAX2 send trunk packets every 20ms
instead of every 50ms.

Tracked down by myself and Bob Wienholt.

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/1.8@355746 65c4cc65-6c06-0410-ace0-fbb531ad65f3