mediatek: mt7622: fix rootfs/ubi detection for Xiaomi AX6S

mediatek: mt7622: fix rootfs/ubi detection for Xiaomi AX6S

By specifying the flag "denx,fit" for partition "kernel", the kernel
try to find rootfs in the same partition during boot. Reality is that
the placement of rootfs is precisely determined by the name of another
partition -"ubi".
It was also found that on some device (for example devices with NAND
chips), the "Denx search engine" manages to find roots at the end of
partition "kernel", but such partition doesn't exist and is empty
there.

Fix this by removing the "denx,fit" flag from partition "kernel". With
this change the original behavior of searchif rootfs in partition "ubi"
is restored.

Signed-off-by: Oleg S <remittor@gmail.com>