Fix the current driver mgmt frame is not respecting the basic rates field
provided by the AP and then unconditionally is using the lowest (1 or 6
Mbps) rate.

For example, if the AP only supported basic rate {24, 36, 48, 54} Mbps,
mt7921 cannot send mgmt frame with the rate not in the group. So,
instead, we pick up the lowest basic rate the AP can support to send.

Fixes: e57b7901 ("mt76: add mac80211 driver for MT7915 PCIe-based chipsets")
Suggested-by: Ryder Lee <ryder.lee@mediatek.com>
Signed-off-by: Sean Wang <sean.wang@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>