MAINTAINERS: update ath6kl

It's not in staging anymore and I'm the current maintainer.

Cc: Luis R. Rodriguez <mcgrof@gmail.com>
Cc: Naveen Singh <nvesing@qca.qualcomm.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>