mac80211: decrease execution of the associated timer

Currently the timer is triggering every two seconds
(IEEE80211_MONITORING_INTERVAL). Decrease the timer to only trigger during
data idle periods to avoid waking up CPU unnecessary. The timer will
still trigger during idle periods, that needs to be fixed later.

There's also a functional change that probe requests are sent only when the
data path is idle, earlier they were sent also while there was activity
on the data path.

This is also preparation for the beacon filtering support. Thanks to
Johannes Berg for the idea.

Signed-off-by: Kalle Valo <kalle.valo@nokia.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>