usb: misc: usbtest: format the data pattern according to max packet size

With this change, the host and gadget doesn't need to agree with transfer
length for comparing the data, since they doesn't know each other's
transfer size, but know max packet size.

Signed-off-by: Peter Chen <peter.chen@freescale.com>
Acked-by: Michal Nazarewicz <mina86@mina86.com>
(Fixed the 'line over 80 characters warning' by Peter Chen)
Tested-by: Peter Chen <peter.chen@freescale.com>
Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Felipe Balbi <balbi@ti.com>