mac80211: add command to get current rssi

Get current rssi (in dBm) from the driver/FW.

Instead of reporting the signal received in the last
rx packet, which might be inaccurate if rx traffic is
low and beacon filtering is enabled, get the signal
from the driver/FW.

Signed-off-by: Victor Goldenshtein <victorg@ti.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>