net80211: add helper functions for determining HT transmit parameters

This adds helper functions to determine a variety of HT related
transmit parameters:

• AMPDU density and size;
• short-GI for 20 and 40MHz;
• Whether 40MHz transmit is allowed;
• Whether HT rates have been negotiated for transmit.

It should be done here rather than drivers having to re-invent the wheel
each time.

This is doubly important for AP modes where each node will hvae
different supported features and this needs to be used when
assembling transmit frames / configuring transmit parameters.

Differential Revision: https://reviews.freebsd.org/D47747

Reviewed by: bz