nios2: Convert pfn_valid to static inline

When compiling with 'W=1' warnings such as the following occur all over
the place:

  ./include/linux/pfn.h:21:22: warning: comparison of unsigned expression
    >= 0 is always true [-Wtype-limits]

which is due to ARCH_PFN_OFFSET being 0 by default
(if CONFIG_NIOS2_MEM_BASE is not changed in the Kconfig). Fix these
warnings by making pfn_valid a static inline function.

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
Signed-off-by: Ley Foon Tan <ley.foon.tan@intel.com>