powerpc/8xxx: Fix in USB device-tree fixup

Fix following issues in USB device-tree fixup:
        - returns when either dr_mode or phy_type not defined.
          This was terminating fix-up when only either property
          was defined in hwconfig string
        - updates dr_mode_type or dr_phy_type with junk value when
          their index is -1. Now these are updated only when their
          respective index is pointing to relevant types
          in modes[] and phys[] array
        - dr_mode_type and dr_phy_type were not NULL for
          each controller

Signed-off-by: Ramneek Mehresh <ramneek.mehresh@freescale.com>
Reviewed-by: York Sun <yorksun@freescale.com>