dwc3: gadget: Clear per endpoint ring buffer when disabling endpoint

Current code is not clearing ring buffer once it is being initialized.
Issue is seen where xferinprogress interrupt is received with
isochronous endpoint due to stale TRB (for isochronous endpoint we
are using LINK TRB), we enqueue new request buffer using update
transfer command. If stale TRB is having HWO bit being set, controller
is trying to execute that particular TRB which results into error on
bus as that request buffer is already being freed or used by other
subsystem. Fix this issue by clearing per endpoint ring buffer when
disabling endpoint.

CRs-Fixed: 782340
Change-Id: I46e6a46400367d6ebf55bdb445a3330c0a4d77e1
Signed-off-by: Mayank Rana <mrana@codeaurora.org>