ramips: simplify ubnt-erx-sfp device definition

Most of the ubnt-erx definition can be reused; the package removals in
DEVICE_PACKAGES have become redundant after d17cb4a68a "ramips: purge
default packages on MT7621".

Signed-off-by: Matthias Schiffer <mschiffer@universe-factory.net>

target/linux/ramips/image/mt7621.mk

@@ -185,15 +185,10 @@ endef
 TARGET_DEVICES += ubnt-erx
 
 define Device/ubnt-erx-sfp
+  $(Device/ubnt-erx)
   DTS := UBNT-ERX-SFP
-  FILESYSTEMS := squashfs
-  KERNEL_SIZE := 3145728
-  KERNEL := $(KERNEL_DTB) | uImage lzma
-  IMAGES := sysupgrade.tar
-  KERNEL_INITRAMFS := $$(KERNEL) | ubnt-erx-factory-image $(KDIR)/tmp/$$(KERNEL_INITRAMFS_PREFIX)-factory.tar
-  IMAGE/sysupgrade.tar := sysupgrade-tar | append-metadata
   DEVICE_TITLE := Ubiquiti EdgeRouter X-SFP
-  DEVICE_PACKAGES := -kmod-mt76 -kmod-rt2x00-lib -kmod-mac80211 -kmod-cfg80211 -wpad-mini -iwinfo kmod-i2c-algo-pca kmod-gpio-pca953x kmod-i2c-gpio-custom
+  DEVICE_PACKAGES += kmod-i2c-algo-pca kmod-gpio-pca953x kmod-i2c-gpio-custom
 endef
 TARGET_DEVICES += ubnt-erx-sfp