Documentation: usb: gadget-testing: add description for depth of queue

Add both bulk and iso depth of queue for sourcesink.
Reviewed-by: Krzysztof Opasiak <k.opasiak@samsung.com>
Signed-off-by: Peter Chen <peter.chen@freescale.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>

Documentation/usb/gadget-testing.txt

@@ -579,6 +579,8 @@ The SOURCESINK function provides these attributes in its function directory:
 	isoc_mult	- 0..2 (hs/ss only)
 	isoc_maxburst	- 0..15 (ss only)
 	bulk_buflen	- buffer length
+	bulk_qlen	- depth of queue for bulk
+	iso_qlen	- depth of queue for iso
 
 Testing the SOURCESINK function
 -------------------------------