usb: dwc3: dbm: Avoid multiple DBM event buffer configuration

DBM event buffer should be configured once for each power
on reset. With this change, any additional call for
dbm_event_buffer_config will return without updating the
registers.

Change-Id: I4a96b3432785d75a5c46c0df7a19901bf700f8f8
Signed-off-by: Bar Weiner <bweiner@codeaurora.org>