ath9k: Move TX99 config option under ath9k debugging

Since ATH9K_TX99 depends on ATH9K_DEBUGFS anyway move it there
such that "make menuconfig" will indent TX99 support below ath9k
debugging.

Signed-off-by: Helmut Schaa <helmut.schaa@googlemail.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>