cmd: mtd: benchmark: use lldiv() instead of 64-bit division

cmd: mtd: benchmark: use lldiv() instead of 64-bit division

As was noted by Heinrich Schuchardt, some SoCs may not support 64-bit
divisions. Fix an issue by using lldiv() instead.

The code assumes that the benchmark never takes more than 4294 seconds
and thus the difference will be less than U32_MAX.

Also replace (speed / 1024) by (speed >> 10) to avoid potential 64-bit
division.

Signed-off-by: Mikhail Kshevetskiy <mikhail.kshevetskiy@iopsys.eu>