usb: dwc3: gadget: remove unnecessary 'dwc' parameter

Endpoint handlers need to know about endpoints, not dwc. If they
really need access to dwc (e.g. for printing error messages) we have a
reference to it tucked inside the endpoint.

This patch has no functional changes, it's simply moving things around.

Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>