nl80211: better describe field in struct nl80211_bss_select_rssi_adjust

The two fields in struct nl80211_bss_select_rssi_adjust did not state
their type or unit. Adding documentation.

Reported-by: Jouni Malinen <j@w1.fi>
Signed-off-by: Arend van Spriel <arend.vanspriel@broadcom.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>