msm: rpm: Initialize the uninitialized variables

sel_mask_ack array will be used to validate the ack received
from rpm. Initialize the sel_mask_ack array to NULL before using it
to make sure that we are comparing the right value read from rpm.

Change-Id: I3dc20bc1594fabf250b98a08461f22482aaa19bf
Signed-off-by: Anji Jonnala <anjir@codeaurora.org>