This reverts commit 27b351c5.

Calling tty_flip_buffer_push on an unopened tty is legal, so the
driver doesn't need track if port has been opened.  Reverting this
allows the entire is_open logic to be removed.
Signed-off-by: NBill Pemberton <wfp5p@virginia.edu>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>