rtwn: update rtwn_get_rates() to separate out the CCK/OFDM and HT rates

The 32 bit bitmap is enough for CCK/OFDM rates and MCS0..15, but
won't work for > MCS15, nor VHT rates.

So, break out the legacy rates and HT rates.

• break the rates and htrates out
• document which calls are looking up basic rates and which care about the rates themselves
• ensure the rate bitmap passed into the rate control firmware call (which isn't enabled yet!) is capped at 28 bits so they don't set the mode field.

Differential Revision: https://reviews.freebsd.org/D47993
Reviewed by: bz, imp