powerpc/spufs: Clear purge status before setting up isolated mode

Currently, we may setup the MFC for isolated mode initilaisation with
the purge still active. This means that DMAs required to perform the
init do not happen.

This change clears the purge status after doing the purge, so that
the isolated init can proceed.

Signed-off-by: Jeremy Kerr <jk@ozlabs.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>