MAINTAINERS: wifi: ath: add Jeff Johnson as maintainer

The "ATHEROS ATH GENERIC UTILITIES" entry shares the same git tree as
the ATH10K, ATH11K, and ATH12K entries which I already maintain, so
add me to that entry as well.

Signed-off-by: Jeff Johnson <jjohnson@kernel.org>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://patch.msgid.link/20241212-ath-maintainer-v1-1-7ea5e86780a8@kernel.org