x86: unify asm/io.h: IO_SPACE_LIMIT

Impact: Cleanup (trivial unification)

Move common define IO_SPACE_LIMIT from <asm/io_*.h> to <asm/io.h>.

Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
Signed-off-by: H. Peter Anvin <hpa@zytor.com>