sched: Fix frequency change checks when affined tasks are migrating

On the 3.18 kernel version and beyond, when the affinity for an
enqueued task changes such that migration is required, the rq
variable gets updated to the destination rq. This means that
check_for_freq_change() skips the source CPU frequency check and
instead double checks the destination CPU. Fix this by using the
src_cpu variable instead.

Change-Id: I14727a34e22c50c9a839007d474802f96a2f49f6
Signed-off-by: Syed Rameez Mustafa <rameezmustafa@codeaurora.org>