Use 'remove_oem_ubi_volume' function to remove OEM UBI volume before upgrade.
This allows to upgrade even if we have OEM firmware on the second partition.
Applies to Linksys MX4200/MX4300/MX5300/MX8500.
Signed-off-by: Paweł Owoc <frut3k7@gmail.com>
Link: https://github.com/openwrt/openwrt/pull/14720
Signed-off-by: Robert Marko <robimarko@gmail.com>