cpufreq: interactive: Fix refcount for migration notification

Current implementation of cpufreq_interactive_enable_sched_input()
returns early if use_sched_input is already enabled. This breaks
refcounting for migration notification registration. It could also
result in failure of registering migration notification after
hotplugging the entire cluster and/or suspend/resume.

Change-Id: I079b2c70b182f696cd8a883f5c8e3a37b5c6d21d
Signed-off-by: Junjie Wu <junjiew@codeaurora.org>