tty: Clean up the ACM driver to CodingStyle

Or at least most of it. There are further clean ups possible and there are
are also thing checkpatch moans about that would be silly to "fix".

Also note some FIXME points found as the cleanup was done.
Signed-off-by: NAlan Cox <alan@linux.intel.com>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>