rtwn: add rtwn_ctl_vhtrate(), some cleanups

• remove the hard-coded ridx values in rate2ridx(), use the RIDX defines for CCK/OFDM rates

• Add a placeholder rtwn_ctl_vhtrate() which returns the dot11rate control rate to use for the given VHT rate index. Since net80211 currently doesn't have any VHT PHY tables, there's no mapping for us to leverage so just return OFDM 12M for now.

• Use the new macro to convert rate index to MCS rate

• Add a printf() in rate2ridx if it's passed a HT/VHT rate.

Differential Revision: https://reviews.freebsd.org/D48098
Reviewed by: bz