usb: phy: mxs: refine mxs_phy_disconnect_line

For non-otg mode, we keep the usage of disconnect line between phy analog
and digital unchanging; for otg mode, at peripheral role, we keep the usage
unchanging too, at host role, the digital part needs to know dp/dm change
to respond device's data pulse when it is at low power mode.

Signed-off-by: Peter Chen <peter.chen@freescale.com>
Signed-off-by: Li Jun <b47624@freescale.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>