powerpc/rtas: Replace magic values with defines

rtas.h already has some nice #defines for RTAS return status
codes - let's use them instead of hard-coded "magic" values!

Signed-off-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Tyrel Datwyler <tyreld@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>