msm: pm2: Update SPC LPM stats correctly

Currently StandalonePC stats are updating incorrectly even
for successful cases. After debug found that compiler has
optimized some of the instructions where the return value
from "msm_pm_power_collapse_standalone()" being compared
incorrectly and always log the stats as failed even for
success cases. Change the "exit_stat" variable to "volatile"
to avoid any optimization.

CRs-Fixed: 433578
Signed-off-by: Murali Nalajala <mnalajal@codeaurora.org>
(cherry picked from commit f59048d29fd75a30cac8c294e068178d9d489af3)

Change-Id: If1f9313f7968e93f5dcd9fb33426496f5091d14a
Signed-off-by: Srivalli Oguri <oguri@codeaurora.org>