ath9k: add a warning to the tx99 config option

Lots of users try random stuff when they encounter any kind of
difficulty. I've had to debug a number of cases where people had enabled
this option for no reason. Hopefully this warning will reduce the number
of useless support cases.

Signed-off-by: Felix Fietkau <nbd@nbd.name>

package/kernel/mac80211/Makefile

@@ -248,7 +248,7 @@ define KernelPackage/ath9k/config
 		depends on PACKAGE_kmod-ath9k
 
        config ATH9K_TX99
-               bool "Enable TX99 support"
+               bool "Enable TX99 support (WARNING: testing only, breaks normal operation!)"
                depends on PACKAGE_kmod-ath9k
 
 	config ATH9K_UBNTHSR