USB: ssu100: fix overrun-error reporting

commit 75bcbf29c284dd0154c3e895a0bd1ef0e796160e upstream.

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

Fixes: 6b8f1ca5581b ("USB: ssu100: set tty_flags in ssu100_process_packet")
Signed-off-by: Johan Hovold <johan@kernel.org>
[bwh: Backported to 3.2:
 - Use tty_port_tty_get() to look up tty_struct
 - Adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>