sched/fair: Limit MAX_PINNED_INTERVAL for more frequent load balancing

Should the system get stuck in a state where load balancing is failing
due to all tasks being pinned, deferring load balancing for up to half
a second may cause further performance problems. Eventually all tasks
will not be pinned and load balancing should not be deferred for a great
length of time.

Change-Id: I06f93b5448353b5871645b9274ce4419dc9fae0f
Signed-off-by: Syed Rameez Mustafa <rameezmustafa@codeaurora.org>
Signed-off-by: Srivatsa Vaddagiri <vatsa@codeaurora.org>