msm: npu: Prevent unpaired power vote/unvote via sysfs node

If there are unpaired power vote/unvote operations via sysfs node,
it is possible that npu power will be turned off unexpected while
npu firmware is still active. This change is to prevent this case.

Change-Id: Icd98b87522ec12ba599dec518fe522056ca40eec
Signed-off-by: Jilai Wang <jilaiw@codeaurora.org>