dahdi_cfg: Warn if idle bits are set on invalid channel type.

Hopefully will save a little time in the future if anyone ever types into
/etc/dahdi/system.conf:

  bchan=1-15,17-30:1101

when they meant:

  cas=1-15,17-30:1101

in the future.

Signed-off-by: Shaun Ruffell <sruffell@digium.com>
Signed-off-by: Russ Meyerriecks <rmeyerriecks@digium.com>