USB: dwc3: Increase event buffer size from 4K to 8K

Controllers prior to 2.30a revision has a bug where Overflow Event may
overwrite an unacknowledged event in the event buffer.  The severity of
the issue depends on the overwritten event type.  Add a warning message
saying that an event is overwritten.  Increase the event buffer size
from 4K to 8K to minimize the chance of event buffer becoming full.

CRs-fixed: 371304
Change-Id: Iad7e9e08426bdf5c3578ea3445e67b21d6dcf038
Signed-off-by: Pavankumar Kondeti <pkondeti@codeaurora.org>