msm: bam_dmux: fix smsm callback racecondition

On targets where the modem can boot first, it is possible for the smsm
callback to be triggered multiple times for the same event as bam_dmux
checks the smsm state asynchronously after registering the callback to
prevent missed events, but the callback could also be called
synchronously for the same event.  To fix, store the last event that was
processed, and if the current event is identical to the last event,
ignore it.

CRs-Fixed: 352600
Change-Id: I5fd84e9f829fbb4e943e38734f9bad75190f3515
Signed-off-by: Jeffrey Hugo <jhugo@codeaurora.org>