ath9k: make the driver specific rate control module optional

ath9k can use minstrel_ht instead, so it makes sense to save some space here.

Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>
diff --git a/drivers/net/wireless/ath/ath9k/Kconfig b/drivers/net/wireless/ath/ath9k/Kconfig
index 35f23bd..ad57a6d 100644
--- a/drivers/net/wireless/ath/ath9k/Kconfig
+++ b/drivers/net/wireless/ath/ath9k/Kconfig
@@ -32,6 +32,14 @@
 
 	  Also required for changing debug message flags at run time.
 
+config ATH9K_RATE_CONTROL
+	bool "Atheros ath9k rate control"
+	depends on ATH9K
+	default y
+	---help---
+	  Say Y, if you want to use the ath9k specific rate control
+	  module instead of minstrel_ht.
+
 config ATH9K_HTC
        tristate "Atheros HTC based wireless cards support"
        depends on USB && MAC80211