m68k: Avoid CONFIG_COLDFIRE switch in uapi header

We should not use any CONFIG switches in uapi headers since
these only work during kernel compilation. They are not defined
for userspace. Let's use the __mcoldfire__ switch from the
compiler here instead.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Greg Ungerer <gerg@linux-m68k.org>