LinuxKPI: 802.11: further fix RSSI calculations

60970a328e280b25b05f1d9a9de1ef91af573c4a did one half of the job
of making rssi relative to nf and numbers for radiotap were fine.
net80211 internally works with .5 dBm units thus we need to apply a

• 2 to the value we pass in to c_rssi; leave a comment explaining.

Note: it is only ifconfig in user space which re-adjust it for printing
or contrib/wpa for calculations. Other applications getting values
from kernel also have to apply the maths.

In collaboration with: J.R. Oldroyd (fbsd opal.com)
Sponsored by: The FreeBSD Foundation
MFC after: 3 days