powerpc/kconfig: Simplify PCI_QSPAN selection

4xx, CPM2 and 8xx cannot be selected at the same time, so
no need to test 8xx && !4xx && !CPM2. Testing 8xx is enough.

Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>