mac80211: disable iwlwifi build for linux 3.18

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

package/kernel/mac80211/Makefile

@@ -785,7 +785,7 @@ endef
 
 define KernelPackage/iwlwifi
   $(call KernelPackage/mac80211/Default)
-  DEPENDS:= +kmod-mac80211 @PCI_SUPPORT +@DRIVER_11N_SUPPORT +iwlwifi-firmware
+  DEPENDS:= +kmod-mac80211 @PCI_SUPPORT +@DRIVER_11N_SUPPORT +iwlwifi-firmware @!LINUX_3_18
   TITLE:=Intel AGN Wireless support
   FILES:= \
 	$(PKG_BUILD_DIR)/drivers/net/wireless/intel/iwlwifi/iwlwifi.ko \