n_tty: Fix throttle for canon lines > 3967 chars

The tty driver will be mistakenly throttled if a line termination
has not been received, and the line exceeds 3967 chars. Thus, it is
possible for the driver to stop sending when it has not yet sent
the newline. This does not apply to the pty driver.

Don't throttle until at least one line termination has been
received.

Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>