powerpc: Minor cleanups for machdep.h

Remove stale declaration of setup_pci_ptrs, aparently from ppc before 2.4.0

Remove #ifdef around struct existance delcaration

Fix spelling of "linear"

Signed-off-by: Milton Miller <miltonm@bga.com>
Signed-off-by: Sonny Rao <sonnyrao@linux.vnet.ibm.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>