usb: chipidea: only get vbus regulator for non-peripheral mode

If the user chooses peripheral mode for this controller, the vbus
regulator doesn't need to get, since the host will supply the vbus,
it can save one vbus pin for other usage.

Signed-off-by: Peter Chen <peter.chen@freescale.com>
Tested-by: Frank Li <frank.li@freescale.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>