s390x/mmu: Use TARGET_PAGE_MASK in mmu_translate_pte()

While ASCE_ORIGIN is not wrong, it is certainly confusing. We want a
page frame address.

Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: David Hildenbrand <david@redhat.com>

target/s390x/mmu_helper.c

@@ -129,7 +129,7 @@ static int mmu_translate_pte(CPUS390XState *env, target_ulong vaddr,
         *flags &= ~PAGE_WRITE;
     }
 
-    *raddr = pt_entry & ASCE_ORIGIN;
+    *raddr = pt_entry & TARGET_PAGE_MASK;
     return 0;
 }