powerpc/mm: Fix verification of MMU_FTR_TYPE_44x

MMU_FTR_TYPE_44x cannot be checked by cpu_has_feature()

Use mmu_has_feature() instead

Fixes: 23eb7f56 ("powerpc: Convert flush_icache_range & friends to C")
Cc: stable@vger.kernel.org
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/ceede82fadf37f3b8275e61fcf8cf29a3e2ec7fe.1602351011.git.christophe.leroy@csgroup.eu