[POWERPC] spufs: fix initial state of wbox file

The wbox channel count of an spu is now initialized
to four for the saved context. This makes it possible
to write to the mailbox right away without waiting
for the SPE to become scheduled first.

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