[MIPS] Add missing braces to pte_mkyoung

Only the version pte_mkyoung for 36-bit pagetables on 32-bit hw was
affected and with this bug being around since November 29, 2004 there
is evidence to suport the assumption it was benign ;-)

Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@helsinki.fi>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>