m68k: merge the non-MMU and MMU versions of swab.h

It is trivial to merge the non-MMU and MMU versions of swab.h.
Without a single file "make headers_install" is broken for m68k
(since each of the sub-varients of swab.h are not installed).

Signed-off-by: Greg Ungerer <gerg@uclinux.org>