usb: dwc3: remove WARN_ON from dwc_stop_active_transfer

Now we are sure that, if res_trans_idx is zero, then endpoint has been
stopped. So it's safe to just return if endpoint is already stopped. No
need to generate warning anymore.

While doing so, it's better to return when res_trans_idx is zero and
decrease one level of indentation.

Signed-off-by: Pratyush Anand <pratyush.anand@st.com>
[ balbi@ti.com: slightly changed commit log ]

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