USB: ci13xxx_udc: Avoid flushing endpoint twice during disable

The ep_disable() function nukes all pending requests before
disabling the endpoint in hardware.  The endpoint is flushed
during nuking.  Don't flush the endpoint again.

CRs-Fixed: 445916
Change-Id: Ib330c67b360b448ec524e6ef0e8c040f04fdf0b8
Signed-off-by: Pavankumar Kondeti <pkondeti@codeaurora.org>