chan_dahdi: Fix "dahdi show channels group" for groups greater than 31.

The variable type used was not large enough to hold a group bit field.
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Merged revisions 380572 from http://svn.asterisk.org/svn/asterisk/branches/1.8


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/11@380575 f38db490-d61c-443f-a65b-d21fe96a405b