powerpc/spufs: remove class_0_dsisr from spu exception handling

According to the CBEA, the SPU dsisr is not updated for class 0
exceptions.

spu_stopped() is testing the dsisr that was passed to it from the class
0 exception handler, so we return a false positive here.

This patch cleans up the interrupt handler and erroneous tests in
spu_stopped. It also removes the fields from the csa since it is not
needed to process class 0 events.

Signed-off-by: Luke Browning <lukebrowning@us.ibm.com>
Signed-off-by: Jeremy Kerr <jk@ozlabs.org>