[POWERPC] spufs: check spu_acquire_runnable() return value

This patch checks return value of spu_acquire_runnable() in
spufs_mfc_write().

Signed-off-by: Akinobu Mita <mita@fixstars.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Arnd Bergmann <arnd.bergmann@de.ibm.com>