cmd: mtd: fix speed measurement in the speed benchmark

cmd: mtd: fix speed measurement in the speed benchmark

The shown speed is inversely proportional to the data size.
See the output:

  spi-nand: spi_nand nand@0: Micron SPI NAND was found.
  spi-nand: spi_nand nand@0: 256 MiB, block size: 128 KiB, page size: 2048, OOB size: 128
  ...
  => mtd read.benchmark spi-nand0 $loadaddr 0 0x40000
  Reading 262144 byte(s) (128 page(s)) at offset 0x00000000
  Read speed: 63kiB/s
  => mtd read.benchmark spi-nand0 $loadaddr 0 0x20000
  Reading 131072 byte(s) (64 page(s)) at offset 0x00000000
  Read speed: 127kiB/s
  => mtd read.benchmark spi-nand0 $loadaddr 0 0x10000
  Reading 65536 byte(s) (32 page(s)) at offset 0x00000000
  Read speed: 254kiB/s

In the spi-nand case 'io_op.len' is not always the same as 'len', thus
we are using the wrong amount of data to derive the speed.

Also make sure we are using 64-bit calculation to get a more precise
results.

Fixes: d246e70cf81d0 ("cmd: mtd: Enable speed benchmarking")
Signed-off-by: Mikhail Kshevetskiy <mikhail.kshevetskiy@iopsys.eu>
Reviewed-by: Miquel Raynal <miquel.raynal@bootlin.com>
Signed-off-by: Michael Trimarchi <michael@amarulasolutions.com>