ath79: correct Siemens WS-AP3610 blocksize

With the wrong blocksize, the rootfs was not positioned on the boundary
of a block, thus breaking the mtdsplit driver.

Signed-off-by: David Bauer <mail@david-bauer.net>

target/linux/ath79/image/generic.mk

@@ -1046,6 +1046,7 @@ define Device/siemens_ws-ap3610
   DEVICE_VENDOR := Siemens
   DEVICE_MODEL := WS-AP3610
   IMAGE_SIZE := 14336k
+  BLOCKSIZE := 256k
   LOADER_TYPE := bin
   LOADER_FLASH_OFFS := 0x82000
   COMPILE := loader-$(1).bin