ramips: add missing kmod-mt7615-firmware for TP-Link RE500 v1

Adding this has been overlooked when rebasing the commit prior to
merge.

Fixes: ba0f4f0 ("ramips: add support for TP-Link RE500 v1")

Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>

target/linux/ramips/image/mt7621.mk

@@ -869,7 +869,7 @@ define Device/tplink_re500-v1
   $(Device/tplink-safeloader)
   DEVICE_MODEL := RE500
   DEVICE_VARIANT := v1
-  DEVICE_PACKAGES := kmod-mt7615e wpad-basic
+  DEVICE_PACKAGES := kmod-mt7615e kmod-mt7615-firmware wpad-basic
   TPLINK_BOARD_ID := RE500-V1
   IMAGE_SIZE := 14208k
 endef