* 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.
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