[POWERPC] cell: Remove DEBUG for SPU callbacks

We don't want SPE programs to be able to flood the kernel log by
invoking the SPE callback handler, so don't enable DEBUG for
spu_callbacks.c by default.

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