usb: chipidea: vbus event may exist before starting gadget

At some situations, the vbus may already be there before starting
gadget. So we need to check vbus event after switching to gadget in
order to handle missing vbus event. The typical use cases are plugging
vbus cable before driver load or the vbus has already been there
after stopping host but before starting gadget.

Signed-off-by: Peter Chen <peter.chen@nxp.com>
Tested-by: Stephen Boyd <stephen.boyd@linaro.org>
Reported-by: Stephen Boyd <stephen.boyd@linaro.org>