Add readme and copy of GPLv2

Move the upstream readme aside, so we can show a sensible readme to a
user of the driver.

The readme explains where I got the code from, what I've done to it,
and how to use the module.

The existing files contain headers indicating they are GPLv2 or later.
In actual fact, they appear to be a copy of cdc-acm.c from the 3.6
kernel with the original copyrights and comments stripped.

Signed-off-by: Joel Stanley <joel@jms.id.au>
Update use of Linux tty api

The following commit changed the api:

commit 05c7cd39907184328f48d3e7899f9cdd653ad336
Author: Jiri Slaby <jslaby@suse.cz>
Date:   Thu Jan 3 15:53:04 2013 +0100

    TTY: switch tty_insert_flip_string

Fixes the following build failures against 3.13:

  In file included from xr21v141x-lnx-3.5-pak/vizzini.c:39:0:
  include/linux/tty_flip.h:29:19: note: expected ‘struct tty_port *’ but argument
  is of type ‘struct tty_struct *’
   static inline int tty_insert_flip_string(struct tty_port *port,
                     ^
  xr21v141x-lnx-3.5-pak/vizzini.c:364:2: warning: passing argument 1 of
  ‘tty_flip_buffer_push’ from incompatible pointer type [enabled by default]
    tty_flip_buffer_push(tty);
    ^
  In file included from xr21v141x-lnx-3.5-pak/vizzini.c:39:0:
  include/linux/tty_flip.h:14:13: note: expected ‘struct tty_port *’ but argument
  is of type ‘struct tty_struct *’
   extern void tty_flip_buffer_push(struct tty_port *port);
               ^

Signed-off-by: Joel Stanley <joel@jms.id.au>
Update use of termios object

The following commit changed the api:

  commit adc8d746caa67fff4b53ba3e5163a6cbacc3b523
  Author: Alan Cox <alan@linux.intel.com>
  Date:   Sat Jul 14 15:31:47 2012 +0100

      tty: move the termios object into the tty

This fixes the following build failure against 3.13:

  xr21v141x-lnx-3.5-pak/vizzini.c: In function ‘xr21v141x_tty_set_termios’:
  xr21v141x-lnx-3.5-pak/vizzini.c:1096:22:
  error: invalid type argument of ‘->’ (have ‘struct ktermios’)
    cflag = tty->termios->c_cflag;

Signed-off-by: Joel Stanley <joel@jms.id.au>