msm: pm-stats: fix the bug in pm stats refactor

Return value of msm_timer_get_sclk_time api is int64_t. Storing the
return value in integer would lead to inconsistencies in pm_stats.

CRs-fixed: 375038
Change-Id: I2c73b387ef01db3a7f71cd1b21b135bea2a475fa
Signed-off-by: Anji Jonnala <anjir@codeaurora.org>