[POWERPC] spufs: clear class2 interrupt status before wakeup

SPU interrupt status must be cleared before handle it.
Otherwise, kernel may drop some interrupt packet.

Currently, class2 interrupt treated like:
 1) call callback to wake up waiting process
 2) mask raised mailbox interrupt
 3) clear interrupt status

I changed like:
 1) mask raised mailbox interrupt
 2) clear interrupt status
 3) call callback to wake up waiting process

Clearing status before masking will make spurious interrupt.
Thus, it is necessary to hold by steps I described above, I think.

Signed-off-by: Masato Noguchi <Masato.Noguchi@jp.sony.com>
Signed-off-by: Arnd Bergmann <arnd.bergmann@de.ibm.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>