bsnmp: wlan: use correct value for rssi

net80211 keeps the rssi value in 0.5bm relative to the noise floor
(see comment in ieee80211_node.h). Do proper maths to get a plain
rssi value back (once further fixes are in net80211).

Sponsored by: The FreeBSD Foundation
MFC after: 3 days
Reviewed by: emaste
Differential Revision: https://reviews.freebsd.org/D50930