m68knommu: make non-MMU page_to_virt() return a void *

The page_to_virt() macro for m68knommu is currently effectively returning
an int type. But the equivilent m68k macro returns a void * virtual address.
Modify the non-MMU macro to return a void * as well (using the __va macro).

This change will remove compiler warnings in common m68k code that use this
macro.

Signed-off-by: Greg Ungerer <gerg@uclinux.org>