If a user switches linemode with sysfs, the VPM would never switch the
companding mode to alaw, and the result would be extreme static on all channels
of the span when the VPM is enabeld.
Now if the deflaw of the span has changed when an echocan is created, the
companding mode of the VPM channel is changed.
Signed-off-by: Shaun Ruffell <sruffell@digium.com>