usb: dwc3: correct set_halt implementation for ep0

set_halt for ep0 is called to stall a deferred control responses by the
gadget.  We already have a function to stall default control endpoint.
This patch points set_halt for ep0 to the already available function.

Signed-off-by: Pratyush Anand <pratyush.anand@st.com>
Signed-off-by: Michel Sanches <michel.sanches@st.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>