powerpc/sstep: Use bitwise instead of arithmetic operator for flags

Fix the following coccinelle warnings:
./arch/powerpc/lib/sstep.c:1090:20-21: WARNING: sum of probable
bitmasks, consider |
./arch/powerpc/lib/sstep.c:1115:20-21: WARNING: sum of probable
bitmasks, consider |
./arch/powerpc/lib/sstep.c:1134:20-21: WARNING: sum of probable
bitmasks, consider |

Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Signed-off-by: Yang Li <yang.lee@linux.alibaba.com>
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/1613811455-2457-1-git-send-email-yang.lee@linux.alibaba.com