net: rfkill: move poll work to power efficient workqueue

This patch moves the rfkill poll_work to the power efficient workqueue.
This work does not have to be bound to the CPU that scheduled it, hence
the selection of CPU that executes it would be left to the scheduler.
Net result is that CPU idle times would be extended, resulting in power
savings.

This behaviour is enabled when CONFIG_WQ_POWER_EFFICIENT is selected.

Cc: "John W. Linville" <linville@tuxdriver.com>
Cc: "David S. Miller" <davem@davemloft.net>
Signed-off-by: Shaibal Dutta <shaibal.dutta@broadcom.com>
[zoran.markovic@linaro.org: Rebased to latest kernel, added commit message.
Fixed workqueue selection after suspend/resume cycle.]
Signed-off-by: Zoran Markovic <zoran.markovic@linaro.org>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>