USB: ohci-omap: defer probe if PHY is missing

Defer probe if PHY is missing. E.g. on Nokia 770 several modules needs
to be loaded to get the PHY going and ohci-omap should wait for those.

Signed-off-by: Aaro Koskinen <aaro.koskinen@iki.fi>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>