rtwn: replace hardcoded rate indices with their names (similar to r289758).

• Use macros to define rate indices; don't hardcode them in code.
• Add method for 'rate' -> 'rate index' conversion

(and array for the opposite).

• Determine if rate is CCK / OFDM via appropriate macro.

Reviewed by: adrian
Differential Revision: https://reviews.freebsd.org/D4837