net80211: correct types for nf and rssi

NF and RSSI should be signed and not unsigned to avoid problems.
Change the type accordingly.

Sponsored by: The FreeBSD Foundation
Approved by: re (gjb)

(cherry picked from commit 4a22cd6c4e5f4fc4a38aa7400742d4005c5ae3de)
(cherry picked from commit 7311128e69b8927d866f9db8c9fa7c5d54eff692)