interactive: avoid calling usecs_to_jiffies() repeatedly

Calling usecs_to_jiffies() repeatedly can be avoided if we store
timer_rate and timer_slack in jiffies from the first place.

Convert 'usecs_to_jiffies' to nothing and
plain timer_* values to 'jiffies_to_usecs(timer_*)'.

Signed-off-by: Park Ju Hyung <qkrwngud825@gmail.com>

Based on commit: https://bitbucket.org/arter97/android_kernel_samsung_exynos7420/commits/3402ebd27c92900acfa06d06dfb1ac2d0483cb46

Conflicts:
	drivers/cpufreq/cpufreq_interactive.c