staging: brcm80211: fill in proper rx rate in mac80211 rx status

The brcmsmac driver supports different rates on 5GHz but this is not
taken into account when providing the rate index in the receive
status information passed to mac80211.

Reviewed-by: Roland Vossen <rvossen@broadcom.com>
Reviewed-by: Pieter-Paul Giesberts <pieterpg@broadcom.com>
Reviewed-by: Henry Ptasinski <henryp@broadcom.com>
Signed-off-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>