target/ppc/mmu_common.c: Convert local variable to bool

In mmu6xx_get_physical_address() ds is used as bool, declare it as
such. Also use named constant instead of hex value.

Signed-off-by: BALATON Zoltan <balaton@eik.bme.hu>
Reviewed-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>