powerpc: Use correct ccr bit for syscall error status

The powerpc implementations of syscall_get_error and
syscall_set_return_value should use CCR0:S0 (0x10000000) for testing
and setting syscall error status.  Fortunately these APIs don't seem
to be used at the moment.

Signed-off-by: Nathan Lynch <ntl@pobox.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>