rtwn: refactor out the r92c path protection decision / configuration

• refactor out the r92c path protection (RTS/CTS) decision
• handle firmware rate control being enabled - if DRVRATE isn't set then the RTSRATE field is ignored and instead RRSR/INIRTS registers are used (and the firmware/hardware will do RTS rate adaptation / retry.)
• when making protection decisions with firmware rate control, default to the channel mode rather than rate index.

This works on RTL8192CU both with firmware rate control and driver
rate control.

Locally tested:

• RTL8192CU, STA - firmware and net80211 rate control

Differential Revision: https://reviews.freebsd.org/D48142