generic: sync mtd rootfs hack with part_bits

In commit ab14364 ("kernel: generic: improve FIT partition parser")
part_bits was bumped to 2 in order to allow up to 3 additional FIT
sub-images mapped into sub-partitions.
This change has to be reflected also in our local patch
420-mtd-set-rootfs-to-be-root-dev.patch
which still assumed part_bits==1 for mtdblock devices in case of
CONFIG_FIT_PARTITION=y.

Fixes: openwrt#9557
Fixes: ab14364 ("kernel: generic: improve FIT partition parser")
Signed-off-by: Daniel Golle <daniel@makrotopia.org>

target/linux/generic/hack-5.10/420-mtd-set-rootfs-to-be-root-dev.patch

@@ -32,7 +32,7 @@ Signed-off-by: Gabor Juhos <juhosg@openwrt.org>
 +		pr_notice("mtd: device %d (%s) set to be root filesystem\n",
 +			  mtd->index, mtd->name);
 +#ifdef CONFIG_FIT_PARTITION
-+		index <<= 1;
++		index <<= 2;
 +#endif
 +		ROOT_DEV = MKDEV(MTD_BLOCK_MAJOR, index);
 +	}