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().
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Merged revisions 398456 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 398457 from http://svn.asterisk.org/svn/asterisk/branches/11
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Merged revisions 398458 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: http://svn.asterisk.org/svn/asterisk/trunk@398462 f38db490-d61c-443f-a65b-d21fe96a405b