ENGR00174734-2 usb: fix bugs that dp and dm are floating at device mode

ENGR00174734-2 usb: fix bugs that dp and dm are floating at device mode

At i.mx6x, the data line (dp and dm) are floating at device mode,
that is to say data line will be any values (0-3.6v).
So if the usb wakeup is enabled, there will be a wakeup interrupt
that causes usb to active mode.

In order to fix this problem well, we need to do below things:
- Need to discharge both dp and dm
- It needs to discharge data line when we switch to device mode and
usb cable is disconnected from the host, but not to disable discharge
after line state is SE0, the reason is that if we do not pulldown
the data line, the line state will be floating again, and possible cause
the wakeup interrupt.
- It needs to disable discharge data line when the usb cable connects at
device mode and usb device is connected at host mode, otherwise it will
affect signal quality.

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