cfg80211: export expected throughput through get_station()

Users may need information about the expected throughput
towards a given peer.
This value is supposed to consider the size overhead
generated by the 802.11 header.

This value is exported in kbps through the get_station() API
by including it into the station_info object.
Moreover, it is sent to user space when replying to the
nl80211 GET_STATION command.

This information will be useful to the batman-adv module
which will use it for its new metric computation.

Signed-off-by: Antonio Quartulli <antonio@open-mesh.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>