mt76: do not use devm API for led classdev

[ Upstream commit 36f7e2b2

 ]

With the devm API, the unregister happens after the device cleanup is done,
after which the struct mt76_dev which contains the led_cdev has already been
freed. This leads to a use-after-free bug that can crash the system.

Signed-off-by: Felix Fietkau <nbd@nbd.name>
Signed-off-by: Sasha Levin <sashal@kernel.org>