mac80211: return the RSSI in dBm

For the sake of speed of calculation and number accuracy,
mac80211 tracks the RSSI in dBm * 16. But it forgot to
divide back by 16 when the RSSI is asked by the driver.

Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>