PM: sleep: No need to check PF_WQ_WORKER in thaw_kernel_threads()

Because PF_KTHREAD is set for all wq worker threads, it is
not necessary to check PF_WQ_WORKER in addition to it in
thaw_kernel_threads(), so stop doing that.

Signed-off-by: Zqiang <qiang.zhang@windriver.com>
[ rjw: Subject and changelog rewrite ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>