usb: dwc3: ep0: fix setup_packet_pending initialization

It just ocurred to me that dwc3 already gives a
really hint of when a setup packet is pending and
that's the SETUP_PENDING TRB Status for EP0 IRQs.

Fix setup_packet_pending initialization based on
that. While at that, also make sure the comment in
gadget.c matches what code is doing.

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