if_iwm - Fix up RSSI calculations for scan results and normal RX operations.

if_iwm - Fix up RSSI calculations for scan results and normal RX operations.

* hard code a noise floor of -96 for now. The noise floor update code
  returns some "interesting" values that I can't map to anything useful
  right now.
* Ensure a default noise floor is set - otherwise the initial scan results
  have a noise floor of '0'.
* Fix up the RSSI calculation to be correctly relative to the noise floor.
  The RSSI routines return an absolute value in dBm - so fix this up.
* Cap RSSI values appropriately.
* Ensure we pass in a 1/2 dB unit value in to net80211.

Taken-From: FreeBSD (SVN r305917)