powerpc/pseries: Really fix the oprofile CPU type on pseries

My previous pach for fixing the oprofile CPU type got somewhat mismerged
(by my fault) when it collided with another related patch. This should
finally (fingers crossed) fix the whole thing.

We make sure we keep the -old- oprofile type and CPU type whenever
one of them was specified in the first pass through the function.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>