USB: keyspan: fix overrun-error reporting

commit 855515a6d3731242d85850a206f2ec084c917338 upstream.

Fix reporting of overrun errors, which are not associated with a
character. Instead insert a null character and report only once.

Fixes: 1da177e4 ("Linux-2.6.12-rc2")
Signed-off-by: Johan Hovold <johan@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>