qcom-cpufreq: Remove per-cpu workqueue

It's no longer a requirement to pin frequency change on the CPU that
is being scaled. Therefore, there is no longer a need for per-cpu
workqueue in qcom-cpufreq. Remove the workqueue.

Change-Id: Ic6fd7f898fa8b1b1226a178b04530c24f0398daa
Signed-off-by: Junjie Wu <junjiew@codeaurora.org>