MFC r305917:

[iwm] fix up RSSI calculations for both 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.