powerpc/mm: remove const type qualifier from function ‘pud_pfn’

Type qualifier on return type is ignored. Remove warning in W=1:

  arch/powerpc/include/asm/book3s/64/pgtable.h:1268:25: error: type qualifiers ignored on function return type [-Werror=ignored-qualifiers]

Signed-off-by: Mathieu Malaterre <malat@debian.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>