wifi: mt76: connac: set fixed_bw bit in TX descriptor for fixed rate frames

Always set the fixed_bw bitfield for fixed rate frames to keep it being
sent with specific bandwidth. Without this change, the bw of fixed rate
frames will still be decided by hardware.

Reported-by: Chank Chen <chank.chen@mediatek.com>
Signed-off-by: Howard Hsu <howard-yh.hsu@mediatek.com>
Signed-off-by: Shayne Chen <shayne.chen@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>