Default to the lowest negotiated rate for mgmt/multicast traffic in 11n mode
Default to the lowest negotiated rate for mgmt/multicast traffic in 11n mode

The current code transmits management and multicast frames at MCS 0.
What it should do is check whether the negotiated basic set is zero (and
the MCS set is not) before making this decision.

For now, simply default to the lowest negotiated rate, rather than
MCS 0. This fixes the behaviour with at least the DLINK DIR-825, which
ACKs but silently ignores block-ack (BA) response frames.

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