[POWERPC] spufs: don't touch suspend bits when purging DMA queue

When we issue a MFC purge request, we may inadvertantly clear the
suspended status.

This change adds the MFC_CNTL_SUSPEND_MASK when we issue a purge
request, so that the suspend bit is masked out.

Signed-off-by: Jeremy Kerr <jk@ozlabs.org>