m68k: rename __iounmap and mark it static

m68k uses __iounmap as the name for an internal helper that is only
used for some CPU types.  Mark it static, give it a better name
and move it around a bit to avoid a forward declaration.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Geert Uytterhoeven <geert@linux-m68k.org>
Acked-by: Geert Uytterhoeven <geert@linux-m68k.org>