ar71xx: use kernel 4.9 by default

Kernel 4.9 support was added about 2 weeks ago and we haven't seen any
major regression so far. This patch was not ported to kernel 4.9, this
needs some additional work:
821-serial-core-add-support-for-boot-console-with-arbitr.patch

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>

target/linux/ar71xx/Makefile

@@ -13,7 +13,7 @@ FEATURES:=usbgadget
 CPU_TYPE:=24kc
 SUBTARGETS:=generic nand mikrotik
 
-KERNEL_PATCHVER:=4.4
+KERNEL_PATCHVER:=4.9
 
 include $(INCLUDE_DIR)/target.mk