usb: chipidea: Fix otg event handler

At OTG work running time, it's possible that several events need to be
addressed (e.g. ID and VBUS events). The current implementation handles
only one event at a time which leads to ignoring the other one. Fix it.

Signed-off-by: Loic Poulain <loic.poulain@linaro.org>
Signed-off-by: Peter Chen <peter.chen@nxp.com>