m68k: Fix indentation by 7 spaces in <asm/io_mm.h>

Indentation should use TABs, not spaces.
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Link: https://lore.kernel.org/r/41ca6ab21459164a52f5f468c32ef09aee7ba3d3.1696602993.git.geert@linux-m68k.org