{cfg,nl}80211: mesh power mode primitives and userspace access

Add the nl80211_mesh_power_mode enumeration which holds possible
values for the mesh power mode. These modes are unknown, active,
light sleep and deep sleep.

Add power_mode entry to the mesh config structure to hold the
user-configured default mesh power mode. This value will be used
for new peer links.

Add the dot11MeshAwakeWindowDuration value to the mesh config.
The awake window is a duration in TU describing how long the STA
will stay awake after transmitting its beacon in PS mode.

Add access routines to:
 - get/set local link-specific power mode (STA)
 - get remote STA's link-specific power mode (STA)
 - get remote STA's non-peer power mode (STA)
 - get/set default mesh power mode (mesh config)
 - get/set mesh awake window duration (mesh config)

All config changes may be done at mesh runtime and take effect
immediately.

Signed-off-by: Marco Porsch <marco@cozybit.com>
Signed-off-by: Ivan Bezyazychnyy <ivan.bezyazychnyy@gmail.com>
Signed-off-by: Mike Krinkin <krinkin.m.u@gmail.com>
[fix commit message line length, error handling in set station]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>