Fixes regression from bb63d03bba (before
v2.7.0). This failed to set the PCM mask on a CAS span when
DAHDI_AUDIO_NOTIFY was not set.
As the first channel of each xbus would be enabled (for
synchronization), a single call may still have passed.
This patch sets the PCM mask on any CAS channel explicitly.
Signed-off-by: Tzafrir Cohen <tzafrir.cohen@xorcom.com>