Fix a bug where DAHDI/Zaptel channels would not properly switch formats when requested

Fix a bug where DAHDI/Zaptel channels would not properly switch formats when requested

Don't offer AST_FORMAT_SLINEAR on DAHDI/Zaptel channels... while it could provide a slight performance benefit, the translation core in Asterisk has some flaws when a channel driver offers multiple raw formats. this fix is much simpler than fixing the translation core to solve that issue (although that will be done later).

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/1.4@186458 65c4cc65-6c06-0410-ace0-fbb531ad65f3