Pass signal strength through, fix units

Pass signal strength through, fix units

The signal strength is currently never used as the only driver reporting
it is nl80211 which uses IEEE80211_RADIOTAP_DB_ANTSIGNAL which is never
populated by the kernel. The kernel will (soon) populate
IEEE80211_RADIOTAP_DBM_ANTSIGNAL instead though, so use that.

Also, since it was never really populated, we can redefine the signal
field to be in dBm units only.

My next patch will also require knowing the signal strength of probe
requests throughout the code (where available), so add it to the
necessary APIs.

Signed-hostap: Johannes Berg <johannes.berg@intel.com>