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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: Bill Pemberton <wfp5p@virginia.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>