wlcore/wl12xx: add global elements to convert hw-rates to standard rates

Rates reported by HW can be different between chip families. Make the
rate-to-idx translation tables private per family and use them in a
common translation function. Add a global element to help determine
which rates are HW HT-rates.

Signed-off-by: Arik Nemtsov <arik@wizery.com>
Signed-off-by: Luciano Coelho <coelho@ti.com>