ath6kl: add an extra band check to ath6kl_wmi_beginscan_cmd()

Dan reported that smatch found a possible issue in ath6kl_wmi_beginscan_cmd()
where we might access sc->supp_rates beyond the end. It shouldn't happen as
ar->wiphy->bands always have just the first two bands set, but add an extra
check just to be sure.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>