IAX2: refactor nativebridge transfer

IAX2: refactor nativebridge transfer

remove triple checking of iaxs[fr->callno]->transferring

reduce indentation.

Reported by: alecdavis
Tested by: alecdavis
alecdavis (license 585)

Review https://reviewboard.asterisk.org/r/2602/
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Merged revisions 391065 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 391084 from http://svn.asterisk.org/svn/asterisk/branches/11

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@391102 65c4cc65-6c06-0410-ace0-fbb531ad65f3