usb: dwc3: core: avoid Overflow events

Now that we're handling so many transfers at a time
and for some dwc3 revisions LPM events *must* be
enabled, we can fall into a situation where too many
events fire and we start receiving Overflow events.

Let's do what XHCI does and allocate a full page for
the Event Ring, this will avoid any future issues.

Cc: <stable@vger.kernel.org> # v4.9
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>