USB: ssu100: fix overrun-error reporting

Fix reporting of overrun errors, which should only be reported once
using the inserted null character.

Fixes: 6b8f1ca5 ("USB: ssu100: set tty_flags in ssu100_process_packet")
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Johan Hovold <johan@kernel.org>