cfg80211: cleanup signal strength units notation

Both cfg80211_rx_mgmt and cfg80211_report_obss_beacon functions send
reports to userspace using NL80211_ATTR_RX_SIGNAL_DBM attribute w/o
any processing of their input signal values. Which means that in
order to match userspace tools expectations, input signal values
for those functions are supposed to be in dBm units.

This patch cleans up comments, variable names, and trace reports
for those functions, replacing confusing 'mBm' by 'dBm'.

Signed-off-by: Sergey Matyukevich <sergey.matyukevich.os@quantenna.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>