net80211: define a type for rssi values

Due to net80211 keeping values in 0.5dBm relative to the noise floor
an int8_t is not good enough to prevent a double wrap around, which
means the reported rssi values can be wrong (see D50928 or likely
a commit in the future for more information).

In order to address the problem and not break the userspace API,
start by defining a type within the kernel and use that. In a
next step we will then update the int8_t to int16_t to avoid the
problem up to the ioctl code. This will then allow us to work
on the the user space API indepedently (see PR 293016 for possible
impact outside the base system).

No functional changes intended.

Sponsored by: The FreeBSD Foundation
Reviewed by: adrian
Differential Revision: https://reviews.freebsd.org/D57021

(cherry picked from commit 96eaa6857ffe009c141c5a6dbea88f96efed9b2d)