remove unneeded explicit channel in dahdi ioctls

This patch removes some cases where the channel number for an ioctl was
passed as a member in a struct rather then through the file descriptor.

The gain setting functions passed around a channel which is always 0,
and thus this parameter is simply dropped.

Review: https://reviewboard.asterisk.org/r/584/


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@254406 65c4cc65-6c06-0410-ace0-fbb531ad65f3