mwl8787: change check for HT40 rate in setting supp rates

check the rates we are using instead of checking our rate
capability in determing rates to pass to fw for
MWL8787_CMD_SET_PEER.

previously, even if we set the channel to HT20, we would
try HT40 rates.

Signed-off-by: Colleen Twitty <colleen@cozybit.com>