callerid: Logic error in checksum processing

Callerid checksum-ing was being handled incorrectly here. When the checksum is
calculated to be 0x00, it will perform 0x100-0x00 which results in 0x100. This
value will then fail the otherwise correct callerid message.

This patch changes the logic to simply add the calculated checksum to the
transmitted 2's compliment checksum.  

Review: https://reviewboard.asterisk.org/r/3356/
(closes issue ASTERISK-23488)
........

Merged revisions 410710 from http://svn.asterisk.org/svn/asterisk/branches/1.8


git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/11@410717 65c4cc65-6c06-0410-ace0-fbb531ad65f3