wcn36xx: move supported_rates to wcn36xx_sta struct

This is were information about supported rates must be stored.
If sta is not available set default rates.

Signed-off-by: Eugene Krasnikov <k.eugene.e@gmail.com>
wcn36xx: remove addresses from wcn36xx struct

Use local variable instead.

Signed-off-by: Eugene Krasnikov <k.eugene.e@gmail.com>