mtd: diskonchip: Cast an operand to prevent potential overflow

There may be a potential integer overflow issue in inftl_partscan().
parts[0].size is defined as "uint64_t"  while mtd->erasesize and
ip->firstUnit are defined as 32-bit unsigned integer. The result of
the calculation will be limited to 32 bits without correct casting.

Fixes: 1da177e4 ("Linux-2.6.12-rc2")
Signed-off-by: Zichen Xie <zichenxie0106@gmail.com>
Cc: stable@vger.kernel.org
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>