powerpc: Avoid comparison of unsigned long >= 0 in __access_ok()

Rewrite function-like macro into regular static inline function to
avoid a warning during macro expansion.

Fix warning (treated as error in W=1):
./arch/powerpc/include/asm/uaccess.h:52:35: error: comparison of unsigned expression >= 0 is always true
   (((size) == 0) || (((size) - 1) <= ((segment).seg - (addr)))))
                                   ^

Suggested-by: Segher Boessenkool <segher@kernel.crashing.org>
Signed-off-by: Mathieu Malaterre <malat@debian.org>
Reviewed-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>