powerpc: Fix duplicate test of MACIO_FLAG_SCCB_ON

Evidently MACIO_FLAG_SCCA_ON was meant.

Signed-off-by: Roel Kluin <roel.kluin@gmail.com>
Acked-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>