MAINTAINERS: wifi: rtw88: change Ping as the maintainer

Yan-Hsuan has been away since 2021 and Ping has been the de facto maintainer
the past year, actively reviewing patches and doing all other maintainer
duties. So fix the MAINTAINERS file to show the current situation.

Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/20230724104547.3061709-2-kvalo@kernel.org