mac80211: Add interface for driver to temporarily disable dynamic ps

This mechanism introduced in this patch applies (at least) for hardware
designs using a single shared antenna for both WLAN and BT. In these designs,
the antenna must be toggled between WLAN and BT.

In those hardware, managing WLAN co-existence with Bluetooth requires WLAN
full power save whenever there is Bluetooth activity in order for WLAN to be
able to periodically relinquish the antenna to be used for BT. This is because
BT can only access the shared antenna when WLAN is idle or asleep.

Some hardware, for instance the wl1271, are able to indicate to the host
whenever there is BT traffic. In essence, the hardware will send an indication
to the host whenever there is, for example, SCO traffic or A2DP traffic, and
will send another indication when the traffic is over.

The hardware gets information of Bluetooth traffic via hardware co-existence
control lines - these lines are used to negotiate the shared antenna
ownership. The hardware will give the antenna to BT whenever WLAN is sleeping.

This patch adds the interface to mac80211 to facilitate temporarily disabling
of dynamic power save as per request of the WLAN driver. This interface will
immediately force WLAN to full powersave, hence allowing BT coexistence as
described above.

In these kind of shared antenna desings, when WLAN powersave is fully disabled,
Bluetooth will not work simultaneously with WLAN at all. This patch does not
address that problem. This interface will not change PSM state, so if PSM is
disabled it will remain so. Solving this problem requires knowledge about BT
state, and is best done in user-space.

Signed-off-by: Juuso Oikarinen <juuso.oikarinen@nokia.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

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