otus(4) - monitor mode fixes, large-mbuf crash fix

• refactor out the rx filter and operating mode code into a separate method.
• add some comments about what's left with setting the operating mode based on what carl9170 does.
• comment out some init from otus_init_mac() - it's no longer needed as it's always init'ed now.
• add debugging and a missing return around a failure to call m_get2() - during monitor mode operation I found RXing of frames > 2k, which fails allocation. I'm sure they're valid (it's configuring 11n RX and receiving 11n frames even though the driver doesn't "do" 11n) and may be A-MSDU; but allocations fail and we should handle that gracefully.

Tested:

• UB82 reference NIC (AR9170 + AR9104 2x2 dual band NIC); STA and monitor mode operation.