[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.

Tested:

• Intel 7260, STA mode

iwm0: <Intel Dual Band Wireless AC 7260> mem 0xf1400000-0xf1401fff irq 17 at device 0.0 on pci2
iwm0: hw rev 0x140, fw ver 16.242414.0, address xx:xx:xx:xx:xx:xx