msm: lpm_levels: Fix incorrect computation of next wakeup time

The next event timer wakeup was incorrectly computed when the hard timer
event doesn't exist. Fix by ensuring the next wakeup is computed only
when a timer event is valid

Change-Id: I40c1f372f84d8f43b2101e37dcfb592193d4b2e0
Signed-off-by: Mahesh Sivasubramanian <msivasub@codeaurora.org>