mt76: mt7915: update the maximum size of beacon offload

Since an in-band discovery frame is offloaded by MCU,
here we enlarge the command size to accommodate the additional content.

Reviewed-by: Ryder Lee <ryder.lee@mediatek.com>
Signed-off-by: Money Wang <Money.Wang@mediatek.com>
Signed-off-by: MeiChia Chiu <MeiChia.Chiu@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>