mt76: mt7915: fix mgmt frame using unexpected bitrate

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>