[POWERPC] spufs: fix incorrect interrupt status clearing in backing mbox stat poll

When doing a poll on the mbox stat file of a swapped-out context, we
clear the class 0 interrupt status, rather than the class 2 interrupt
status.

This change corrects the poll operation to clear the correct interrupt.

Signed-off-by: Jeremy Kerr <jk@ozlabs.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>