Some gadget driver (like uac1) will try to enable endpoint again even
the ep is not empty, it will cause the ep reset again and may affect
the dTD list which has already queued.

It returns -EBUSY immediately, and indicate the endpoint is in use.
In this way, the ep's behavior will not be affected, and the gadget
driver is also notified.

Cc: Xuebing Wang <xbing6@gmail.com>
Signed-off-by: Peter Chen <peter.chen@freescale.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>