udc: net2280: Fix net2280_disable

A reset e.g. calling ep_reset_338x() can happen while endpoints
are enabled. The ep_reset_338x() sets ep->desc = NULL to mark
endpoint being invalid. A subsequent call of net2280_disable will
fail and return -EINVAL to parent function usb_ep_disable(),
which will fail, too, and do not set the member ep->enabled = false.

See:
https://elixir.bootlin.com/linux/v5.0-rc5/source/drivers/usb/gadget/udc/core.c#L139



This fix ignores dp->desc and allows net2280_disable() to succeed.
Subsequent calls to usb_ep_enable()/usb_ep_disable() succeeds.

Acked-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Guido Kiener <guido.kiener@rohde-schwarz.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>