powerpc/mce: use early_cpu_to_node() in mce_init()

cpu_to_node() is not yet available (setup_arch() is called before
setup_per_cpu_areas() by start_kernel()).

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20220711030653.150950-1-npiggin@gmail.com