usb: misc: usbtest: using the same data format among write/compare/output

Using the same data format "buf[j] = (u8)(i + j)" among
write, compare buf, and console output stage.

Acked-by: Michal Nazarewicz <mina86@mina86.com>
Signed-off-by: Peter Chen <peter.chen@freescale.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>