USB: ci13xxx_udc: Flush ep0 even when queue is empty

The endpoint must be flushed before retiring pending requests. The
current code skips flushing an endpoint if there are no pending
requests.  Flushing ep0 for every setup packet is some how resolving
the race between transfer completion interrupt and transfer descriptors
updates to the system memory in the controller.  Always allow flushing
ep0 endpoint.  This patch resolves USB disconnect issue during RNDIS
high bandwidth tests.

CRs-Fixed: 466538
Change-Id: Ia89b2dc8ff831641dafb91196e3af5271ac66cfd
Signed-off-by: Pavankumar Kondeti <pkondeti@codeaurora.org>