wifi: mt76: mt7921: fix invalid remain_on_channel duration

round_jiffies_up() may change the duration reported from chip. We should
take the real timeout for current channel privilege.

Fixes: 034ae28b ("wifi: mt76: mt7921: introduce remain_on_channel support")
Signed-off-by: Deren Wu <deren.wu@mediatek.com>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>