usb: xhci-dwc3: Power on USB PHY before using

It is wrong that expect .phy_init() to also power on the PHY. Therefore,
explicitly, call generic_phy_power_on() after generic_phy_power_init() in
order to power on PHY before using it.

Signed-off-by: Vignesh R <vigneshr@ti.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>