MIPS: Fix CONFIG_FLATMEM version of pfn_valid()

For systems which do not define PHYS_OFFSET as 0 pfn_valid() may falsely
have returned 0 on most configurations.  Bug introduced by commit
752fbeb2e3555c0d236e992f1195fd7ce30e728d (linux-mips.org) rsp.
6f284a2c (kernel.org) titled "[MIPS]
FLATMEM: introduce PHYS_OFFSET."

Signed-off-by: Ralf Baechle <ralf@linux-mips.org>