doc: usb: gadget-testing: using the updated testusb.c

testusb.c at http://www.linux-usb.org/usbtest/

 is out of date,
using the one at the kernel source folder.

Cc: <stable@vger.kernel.org>
Signed-off-by: Peter Chen <peter.chen@freescale.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>