usb: otg: OMAP4430: Fixing the omap4430_phy_init function

omap4430_phy_init() function can be called with no device pointer,
to powerdown the PHY during board init when USB is disabled.
Fix the function accordingly.

Signed-off-by: Hema HK <hemahk@ti.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>