mtd: parsers: tplink_safeloader: fix uninitialized variable bug

On 64 bit systems, the highest 32 bits of the "offset" variable are
not initialized.  Also the existing code is not endian safe (it will
fail on big endian systems).  Change the type of "offset" to a u32.

Fixes: aec4d5f5ffd0 ("mtd: parsers: add TP-Link SafeLoader partitions table parser")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Rafał Miłecki <rafal@milecki.pl>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/Y1gCALFWXYYwqV1P@kili