usb: dwc3: gadget: don't prestart interrupt endpoints

Because interrupt endpoints usually transmit such
small amounts of data, it seems pointless to prestart
transfers and try to get speed improvements. This
patch also sorts out a problem with CDC ECM function
where its notification endpoint gets stuck in busy
state and we continuously issue Update Transfer
commands.

Fixes: 8a1a9c9e ("usb: dwc3: gadget: start transfer on XFER_COMPLETE")
Signed-off-by: Felipe Balbi <balbi@ti.com>