usb: gadget: dummy_hcd: add sg support

This patch adds sg support to dummy_hcd. It seems that uas is not able
to work with a hcd which does not support sg only based transfers.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Acked-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Felipe Balbi <balbi@ti.com>