The round robin routing routine in chan_misdn.c is broken.

The round robin routing routine in chan_misdn.c is broken.

it rotates between ports but never checks the channels in the ports.

i have extensivly tested it and verified it works on 1 upto 4 ports.
before the patch only 1 out of each port was used now all are used as
expected.

(closes issue ASTERISK-18413)
Reported by: irroot
Tested by: irroot
Reviewed by: irroot

Review: https://reviewboard.asterisk.org/r/1410/

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/1.8@336166 65c4cc65-6c06-0410-ace0-fbb531ad65f3