MAINTAINERS: update mt76 reviewers

Roy no longer works here. Time to say goodbye, my friend.

Signed-off-by: Ryder Lee <ryder.lee@mediatek.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/c171e0dfce9f2dad5ca6935eaf6004117f82e259.1587195398.git.ryder.lee@mediatek.com