ENGR00162583-3 usb header: avoid class driver access register after usb is off

ENGR00162583-3 usb header: avoid class driver access register after usb is off

- An well-behavior class driver should disable their endpoints
after being notified disconnect with host, we use all endpoints
are stopped (ep->stopped)  to indicates the class
driver will not visit device driver any more.
the ep-stopped will be initialized as 1 for non-control endpoint
it will be 0 after fsl_ep_enable, and be 1 after fsl_ep_disable.
Where is a non-sleep wait routine at disconnect event for waiting all
endpoints are stopped
- Some controller's (like i.mx6q) DP will change from J
to SE0 slowly after the cable disconnects with host, in that case there
will be a wakeup interrupt after driver enables the wakeup interrupt.
For i.mx6q, there is a discharge routine for DP after the disconnection.
- Should not wait vbus to low during first otg switch, as the wait will
be timeout when the usb cable is connecting to host.

Signed-off-by: Peter Chen <peter.chen@freescale.com>