USB: ci13xxx_udc: Flush ep0IN and ep0OUT endpoint from ep_dequeue

Flush control endpoints from ep_dequeue and set request completion
to NULL once calling completion handler of function driver. This will
make sure that all control endpoint requests are flushed in disconnect.

CRs-Fixed: 373672
Change-Id: I7506c2e53d13f137ae6f222a06fbcc0dee77f68e
Signed-off-by: Mayank Rana <mrana@codeaurora.org>