drivers: cpuidle: msm: Use 64 bit type for sclk

'us' can overflow and can potentially cause unexpected wakeup.

Change it to uint64_t.

Change-Id: I943cbc9d62268ca073e388c287e3b180c0eaa8e3
Signed-off-by: Girish S Ghongdemath <girishsg@codeaurora.org>
Signed-off-by: Deven Patel <cdevenp@codeaurora.org>