MAINTAINERS: hand over ath9k maintainership to Toke

In practise I have been the only maintainer for ath9k for some time now but I
don't really have time for it. Luckily Toke is willing to look after so mark
him as maintainer. Thanks Toke!

Signed-off-by: Kalle Valo <kvalo@kernel.org>
Acked-by: Toke Høiland-Jørgensen <toke@toke.dk>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/20220128135841.1926-3-kvalo@kernel.org