usb: gadget: f_sourcesink: add queue depth

Add queue depth for both iso and bulk transfer, with more queues, we
can do performance and stress test using sourcesink, and update g_zero
accordingly.

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>