usb: dwc3: remove num_event_buffers

We never, ever route any of the other event buffers
so we might as well drop support for them.

Until someone has a real, proper benefit for
multiple event buffers, we will rely on a single
one. This also helps reduce memory footprint of
dwc3.ko which won't allocate memory for the extra
event buffers.

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