mtd: bcm47xxpart: improve handling TRX partition size

When bcm47xxpart finds a TRX partition (container) it's supposed to jump
to the end of it and keep looking for more partitions. TRX and its
subpartitions are handled by a separate parser.

The problem with old code was relying on the length specified in a TRX
header. That isn't reliable as TRX is commonly modified to have checksum
cover only non-changing subpartitions. Otherwise modifying e.g. a rootfs
would result in CRC32 mismatch and bootloader refusing to boot a
firmware.

Fix it by trying better to figure out a real TRX size. We can securely
assume that TRX has to cover all subpartitions and the last one is at
least of a block size in size. Then compare it with a length field.

This makes code more optimal & reliable thanks to skipping data that
shouldn't be parsed.

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>