[POWERPC] spufs: Don't leak kernel stack through an empty {i,m}box_info read

Based on an original patch from Arnd Bergmann
<arnd.bergmann@de.ibm.com>

If there's no entry in the mailbox, then a read on the _info file will
return data from an uninitialised variable.

This change returns EOF if there's no mailbox info available instead.

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