mt76: add mt76_default_basic_rate more devices can rely on

add mt76_default_basic_rate to mt76 core more devices can rely on
to send frames which require the rate in the basic rates.

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>