mt76: connac: fix the maximum interval schedule scan can support

[ Upstream commit abded041 ]

Maximum interval (in seconds) for schedule scan plan supported by
the offload firmware can be U16_MAX.

Signed-off-by: Sean Wang <sean.wang@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>
Signed-off-by: Sasha Levin <sashal@kernel.org>