cmd: mtd: avoid unintentional integer overflow

cmd: mtd: avoid unintentional integer overflow

mtd dump beyond 4 GiB will show incorrect results.

Multiplying two u32 will yield a u32. Add a missing cast.

Fixes: 5db66b3aee6f ("cmd: mtd: add 'mtd' command")
Addresses-Coverity-ID: 477205 ("Unintentional integer overflow")
Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/all/20240111073155.19545-1-heinrich.schuchardt@canonical.com
Signed-off-by: Dario Binacchi <dario.binacchi@amarulasolutions.com>