[POWERPC] TLB insertion cleanup

This patch changes handling return value of ppc_md.hpte_insert() into
the same way as __hash_page_*().

Signed-off-by: Kou Ishizaki <kou.ishizaki@toshiba.co.jp>
Acked-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>