clang compiler warnings: Fix -Wbitfield-constant-conversion warning

In chan_iax2, we attempt to assign a -1 to a bitfield. This gets caught by
clang, as it will truncate the -1 to a 1 implicitly.

Instead, we just assign the value a '1'.

Review: https://reviewboard.asterisk.org/r/4537/

ASTERISK-24917
Reported by: dkdegroot
patches:
  rb4537.patch submitted by dkdegroot (License 6600)
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Merged revisions 433683 from http://svn.asterisk.org/svn/asterisk/branches/11
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Merged revisions 433684 from http://svn.asterisk.org/svn/asterisk/branches/13


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@433686 65c4cc65-6c06-0410-ace0-fbb531ad65f3