usb: dwc2/gadget: break infinite loop in endpoint disable code

This patch fixes possible freeze caused by infinite loop in interrupt
context.

Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Robert Baldyga <r.baldyga@samsung.com>
Cc: stable <stable@vger.kernel.org> # 3.16
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>