usb: host: ohci-generic: initialize PHY only when found

Call generic_phy_init() only when a PHY was found.
This will avoid a crash if no "phys" property is found in DT.

Signed-off-by: Patrice Chotard <patrice.chotard@st.com>
Reported-by: Patrick Delaunay <patrick.delaunay@st.com>