usb: dwc3: gadget: simplify __dwc3_gadget_ep_queue()

Many of the comments in that function are really
outdated and don't match what the driver is
doing. Moreover, recent patches combined programming
model for all non-control endpoints, this gives us
an opportunity to get rid of our special cases in
__dwc3_gadget_ep_queue().

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