iwx: clean up / document noise floor and RSSI fetching

• Document what iwx_rxmq_get_signal_strength() is doing in a comment, noting what the firmware returns and what math is being done on it to turn it into a dBm value.

• Document what iwx_get_noise() is supposed to do, that we can't just go do math with log numbers like we're doing, but also that we're seeing zeros in this firmware (AX210), which may mean we're decoding using the wrong structs.

• Swizzle around the RSSI calculation as a function, add min/max RSSI values, and calculate RSSI against the noise floor.

• And handle the lowest noise floor value - it can't be -127dBm as that will throw things off. Cap it at -100dBm which is a little lower than the thermal noise floor at 20MHz (-98dBm), but it matches IWX_MIN_DBM.

Differential Revision: https://reviews.freebsd.org/D53780