cpufreq: interactive: Correctly reschedule timer for slack_only case

Slack timer's expire field was not correctly initialized if slack_only
is true in cpufreq_interactive_timer_resched(). This causes both
compilation warning and functional breakage.

Fix expire field by setting it properly.

Change-Id: I2f8c454d63626876522c163eb8d3c5d1c8adfd51
Signed-off-by: Junjie Wu <junjiew@codeaurora.org>