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testusb.c at http://www.linux-usb.org/usbtest/ is out of date,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>

Documentation/usb/gadget-testing.txt

@@ -237,9 +237,7 @@ Testing the LOOPBACK function
 -----------------------------
 
 device: run the gadget
-host: test-usb
-
-http://www.linux-usb.org/usbtest/testusb.c
+host: test-usb (tools/usb/testusb.c)
 
 8. MASS STORAGE function
 ========================
@@ -586,9 +584,8 @@ Testing the SOURCESINK function
 -------------------------------
 
 device: run the gadget
-host: test-usb
+host: test-usb (tools/usb/testusb.c)
 
-http://www.linux-usb.org/usbtest/testusb.c
 
 16. UAC1 function
 =================