rfkill: prevent unnecessary event generation

Prevent unnecessary rfkill event generation when the state has
not actually changed. These events have to be delivered to
relevant userspace processes, causing these processes to wake
up and do something while they could as well have slept. This
obviously results in more CPU usage, longer time-to-sleep-again
and therefore higher power consumption.

Signed-off-by: Vitaly Wool <vitalywool@gmail.com>
Signed-off-by: Mykyta Iziumtsev <nikita.izyumtsev@gmail.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>