usbd msc clear request support

[alexrayne]

This patch includes: !fix usbd stop ep !usbd:dwc_otg:OUT prematured urb

and also provides enhanced URB cancel in DWC backend - now canceling URB can detects that ep is in transmition process and abort and flush it. MSC class implements support Clear Feature Endpoint request - that cancels current transfered URB.

it is tested only in bulk IN endpoint now.