powerpc/cell/OProfile: Fix on-stack array size in activate spu profiling function

The size of the pm_signal_local array should be equal to the
number of SPUs being configured in the array.  Currently, the
array is of size 4 (NR_PHYS_CTRS) but being indexed by a for
loop from 0 to 7 (NUM_SPUS_PER_NODE).  This could potentially
cause an oops or random memory corruption since the pm_signal_local
array is on the stack.  This fixes it.

Signed-off-by: Carl Love <carll@us.ibm.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>