powerpc: Fix wrong error code from ppc32 select syscall

This patch was submitted, discussed, and eventually Acked by everyone, yet
still isn't in the tree.  See:

http://patchwork.ozlabs.org/patch/1240/



Signed-off-by: Josh Boyer <jwboyer@linux.vnet.ibm.com>
Cc: Arnd Bergmann <arnd@anrdb.de>
Cc: Paul Mackerras <paulus@samba.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>