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>

arch/powerpc/include/asm/book3s/64/pgtable.h

@@ -1304,7 +1304,7 @@ static inline int pgd_devmap(pgd_t pgd)
 }
 #endif /* CONFIG_TRANSPARENT_HUGEPAGE */
 
-static inline const int pud_pfn(pud_t pud)
+static inline int pud_pfn(pud_t pud)
 {
 	/*
 	 * Currently all calls to pud_pfn() are gated around a pud_devmap()