usb: dwc3: decrease event buffer size

Currently we're allocating an entire page to
serve as our event buffer. Provided our events
are 4 bytes long, it's very unlikely we will
even trigger 1k events at once.

Even in the worst case scenario where every
endpoint triggers one event and we still have
a couple of error events, that would still
be less than 40 events.

In order to cope with future versions of the
IP which could (or could not) increase the
amount of possible events to trigger
simultaneously, we're using an arbitrary size
of 64 events for our event buffer.

We're saving 3840 bytes by doing so.

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