powerpc/mm: Fix typo in set_pte_at()

This fixes the typo about the _PAGE_PTE in set_pte_at() by changing
"tryint" to "trying to".

Signed-off-by: Gavin Shan <gwshan@linux.vnet.ibm.com>
Acked-by: Balbir Singh <bsingharora@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>