Remove some redundant isync instructions.

enable_64b_mode() already does an isync, so there is no need to do it again.
Signed-off-by: MOKUNO, Masakazu <mokuno@sm.sony.co.jp>
Acked-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>