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/Makefile b/drivers/net/wireless/ath/ath9k/Makefile
index 4555e99..aca0162 100644
--- a/drivers/net/wireless/ath/ath9k/Makefile
+++ b/drivers/net/wireless/ath/ath9k/Makefile
@@ -5,8 +5,8 @@
 		recv.o \
 		xmit.o \
 		virtual.o \
-		rc.o
 
+ath9k-$(CONFIG_ATH9K_RATE_CONTROL) += rc.o
 ath9k-$(CONFIG_PCI) += pci.o
 ath9k-$(CONFIG_ATHEROS_AR71XX) += ahb.o
 ath9k-$(CONFIG_ATH9K_DEBUGFS) += debug.o