[otus] implement monitor mode.

• break out the operating mode and rx filter into new functions, rather than them being hard-coded
• if we're in sniffer mode or not associated, set the BSS MAC to all zero, rather than relying on a chip reset to do it for us
• add comments about .. how interestingly buggy the chip is.

Tested:

• AR9170 + AR9102, STA+monitor mode

Obtained from: linux carl9170 (general chip workings, constant definitions)