dwc3: Increase event buffer size to 8KB from 256 bytes

Commit 5da93478 ("usb: dwc3: decrease event buffer size") has
decreased controller's event buffer size to 256 bytes which results
into multiple overflow events. Receiving of overflow events suggests
that software is not able to process received events fast enough
which results into losing of those events. Fix this issue by
increasing event buffer size to 8KB.

Change-Id: I37f6203f0b6d87098d57b2ce0ba8a3a0b8ef5bd2
Signed-off-by: Mayank Rana <mrana@codeaurora.org>