um: define set_pte_at() as a static inline function, not a macro

When defined as macro, the mm argument is unused and subsequently the
variable passed as mm is considered unused by the compiler. This fixes
a build warning.

Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Reviewed-by: Geert Uytterhoeven <geert@linux-m68k.org>
Reviewed-by: Anton Ivanov <anton.ivanov@cambridgegreys.com>
Acked-by: Anton Ivanov <anton.ivanov@cambridgegreys.com>
Signed-off-by: Richard Weinberger <richard@nod.at>