usb: misc: usbtest: delete useless memset for urbs array

The element of urbs array will be initialized at below code
at once.

	for (i = 0; i < param->sglen; i++) {
		urbs[i] = iso_alloc_urb(udev, pipe, desc,
					param->length, offset);

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