cpufreq: interactive governor: Delete boost timer on CPUFREQ_GOV_STOP.

Concurrent execution of CPUFREQ_GOV_STOP and interactive_boost_timer()
should be avoided.
Also, interactive_boost() should follow CPUFREQ_GOV_STOP on
flush_work().

Change-Id: I53e897d5ee1834f5cbf0778dd48fc604ec3ea34d
Signed-off-by: Oleg Matcovschi <oleg.matcovschi@ti.com>