mac80211: track AP's beacon rate and give it to the driver

Track the AP's beacon rate in the scan BSS data and in the
interface configuration to let the drivers know which rate
the AP is using. This information may be used by drivers,
in our case to let the firmware optimise beacon RX.

Signed-off-by: Alexander Bondar <alexander.bondar@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>