ramips: pax1800-lite: do not attach both ubi partitions on boot

ramips: pax1800-lite: do not attach both ubi partitions on boot

The dual-boot mechanism depends on the fact that the bootloader specifies
the ubi.mtd= of the currently active slot. And the Linux is expected to
only attach the specified ubi-partition. Otherwise the kernel will use the
"rootfs" partition of the initially attached ubi partition as its root
partition. Which is of course wrong when the kernel parameter specified
ubi.mtd=firmware2.

Fixes: c7c54f313425 ("ramips: add support for Plasma Cloud PAX1800-Lite")
Signed-off-by: Sven Eckelmann (Plasma Cloud) <se@simonwunderlich.de>
Link: https://github.com/openwrt/openwrt/pull/23857
Signed-off-by: Jonas Jelonek <jelonek.jonas@gmail.com>