usb: dwc3: gadget: fix ep->maxburst for ep0

dwc3_gadget_set_ep_config expects maxburst as incremented by 1. So, by
default initialize ep->maxburst to 1 for ep0.

Cc: <stable@vger.kernel.org>
Signed-off-by: Pratyush Anand <pratyush.anand@st.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>