powerpc/86xx: mpc86xx_hpcn: Call uli_init() instead of explicit ppc_md assignment

After calling fsl_pci_assign_primary(), it is possible to use uli_init() to
conditionally initialize ppc_md.pci_exclude_device callback based on the
uli1575 detection.

Signed-off-by: Pali Rohár <pali@kernel.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20230409000812.18904-8-pali@kernel.org