m68knommu: Correct page_to_phys when PAGE_OFFSET is non-zero.

The definition of page_to_phys for nommu produces an incorrect value when
PAGE_OFFSET is non-zero.  The nommu version of page_to_pfn works correctly
for non-zero PAGE_OFFSET, so use that instead.

Signed-off-by: Steven King <sfking@fdwdc.com>
Signed-off-by: Greg Ungerer <gerg@uclinux.org>