Revert "USB: quatech2: only write to the tty if the port is open."

This reverts commit 27b351c5546008c640b3e65152f60ca74b3706f1.

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>