devicestate.c: Minor tweaks

* In ast_state_chan2dev() use ARRAY_LEN() instead of a sentinel value in
chan2dev[].

* Fix some comments in chan_iax2.c.
........

Merged revisions 422661 from http://svn.asterisk.org/svn/asterisk/branches/13


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@422663 65c4cc65-6c06-0410-ace0-fbb531ad65f3