[POWERPC] spufs: Fix update of mailbox status register during backed wbox write

When a process writes into the inbound spu mailbox (wbox) while the
context is saved, we accidentally break the contents of the mb_stat_R
register by clearing other entries of the mailbox status register. This
can cause the user side to hang.

This change fixes the problem by only altering the appropriate bits
of the mailbox status register during a backing-store write.

Signed-off-by: Arnd Bergmann <arnd.bergmann@de.ibm.com>
Signed-off-by: Jeremy Kerr <jk@ozlabs.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>