USB: ci13xxx_udc: Don't flush inactive endpoints

The endpoint must be flushed prior to retiring pending requests.
This is done during disabling an endpoint.  Don't flush an endpoint
if there are no pending requests.  According databook flushing
endpoint may take large amount of time.  This optimization will
make USB disconnect/reset processing a little bit faster.

(cherry picked from commit 58db2c1f94451d7a352d2ad1a1b1e9732f4f0a11)

CRs-Fixed: 445916
Change-Id: I2a8b185d7f6f295485bc1d1b1794f42471149491
Signed-off-by: Pavankumar Kondeti <pkondeti@codeaurora.org>