usb: dwc3: gadget: simplify dwc3_gadget_ep_queue()

By moving our sanity checks our internal function
__dwc3_gadget_ep_queue() we can simplify the
externally visible API while also making sure that
callers of __dwc3_gadget_ep_queue() also make use of
the same checks.

Signed-off-by: Felipe Balbi <balbi@ti.com>