chan_iax2: Reduce indentation in __attempt_transmit().

* Reduce indentation in __attempt_transmit().

* Don't update the static last error time variable every time in
__schedule_action() and socket_read().
........

Merged revisions 398456 from http://svn.asterisk.org/svn/asterisk/branches/1.8


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/11@398457 f38db490-d61c-443f-a65b-d21fe96a405b