mediatek: filogic: fix sysupgrade for Wavlink WL-WNT100X3

mediatek: filogic: fix sysupgrade for Wavlink WL-WNT100X3

The stock layout of the WL-WNT100X3 splits the SPI-NAND into
bl2/u-boot-env/factory/fip/ubi/hw and the sysupgrade image is a
sysupgrade-tar that has to be unpacked into the "ubi" partition.

The board was however added to the default_do_upgrade() case, which
writes the image to ${PART_NAME:-image}, i.e. to the "firmware" MTD
partition that does not exist on this device:

  find_mtd_index firmware -> []
  mtd: Could not open mtd device: firmware

mtd fails, do_stage2 exits before flashing anything and the board
reboots into the old firmware without a visible error, so every
OpenWrt-to-OpenWrt sysupgrade looks like a silent no-op. Only the
initial installation works, since that one is performed by the sysupgrade
of the vendor firmware.

Drop the entry so the board falls through to the generic
nand_do_upgrade() case with CI_UBIPART defaulting to "ubi", like the
identically partitioned WL-WN586X3B.

The WL-WNT100X3 (OpenWrt U-Boot layout) variant uses fit_do_upgrade()
and is not affected.

Fixes: 829d432ecda3 ("mediatek: add support for Wavlink WL-WNT100X3")
Signed-off-by: Fil Dunsky <filipp.dunsky@gmail.com>
Link: https://github.com/openwrt/openwrt/pull/24684
Signed-off-by: Jonas Jelonek <jelonek.jonas@gmail.com>