usb: dwc3: gadget: retire LST bit completely

The only endpoint which actually requires LST bit
and XferComplete is ep0/1. Let's save some time by
completely removing LST bit support and
XferComplete.

This simplifies and consolidates endpoint handling
for all other 3 transfer types while also avoiding
extra interrupts.

Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>