msm: bam_dmux: fix ack race condition

If bam_dmux votes for A2 powerdown, and then votes for A2 powerup within
a short peroid of time, bam_dmux may intrepret the powerdown ack from A2
as the power up ack which would break the A2 PC protocol.  Add an
explicit check to make sure that A2 powerup vote waits for any
previous powerdown acks.

Change-Id: I573c76034c2273da2b95891fab6a9a179bd35805
CRs-Fixed: 322305
Signed-off-by: Jeffrey Hugo <jhugo@codeaurora.org>