usb: phy: initialize the notifier when add a new phy

usb: phy: initialize the notifier when add a new phy

We need to initialize the notifer before use it.

So lets initialize it when add a new phy device to
reduce the code redundancy.

Signed-off-by: Neil Zhang <zhangwm@marvell.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>