mt76: mt7915: fix an off-by-one bound check

[ Upstream commit d45dac07 ]

The bounds check on datalen is off-by-one, so fix it.

Signed-off-by: Ryder Lee <ryder.lee@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>
Signed-off-by: Sasha Levin <sashal@kernel.org>