usb: gadget: dummy_hcd: emulate sending zlp in packet logic

currently, when a zlp flag is set and an urb/usb_request
buffer is filled without a short packet, transfer() leaves
its status at -EINPROGRESS and does not rescan for short
packet.

In a scenario where ep.maxpacket bytes are copied,
URB_ZERO_PACKET is set, urb buffer is filled and usb_request
buffer is not, transfer() returns with an urb with
-EINPROGRESS status, which dummy_hcd treats as incomplete
transfer.

Check for zlp and rescan appropriately.

Signed-off-by: Igor Kotrasinski <i.kotrasinsk@samsung.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>