mwl8787: convert 5ghz rate indexes

As Thomas pointed out, CCK rates aren't used in 5ghz,
so the rate index returned by the firmware needs to be shifted
properly to account for this.

Signed-off-by: Bob Copeland <me@bobcopeland.com>
mwl8787: skip 22 mbps rate in 5ghz rate array

Since the 22 mbps rate was added we were including it
in the 5ghz rateset.

Signed-off-by: Bob Copeland <me@bobcopeland.com>