dahdi: Stop tones on channel when updating tone zone.

If a channel is currently playing a tone when the tone zone is updated, the
existing tone zone could be freed while the channel keeps a reference to the
current tone (curtone) that points into the freed zone.

If the newly freed tone is then modified, there was a window where it was
possible to  corrupt 'struct dahdi_chan' (by overrunning swritechunk[])
resulting in a "BUG: unable to handle kernel paging request at virtual address"
panic in the context of __dahdi_transmit_chunk().

Reported-and-Tested-by: Matt Behrens <matt@zigg.com>
Signed-off-by: Shaun Ruffell <sruffell@digium.com>
Signed-off-by: Russ Meyerriecks <rmeyerriecks@digium.com>