USB: keyspan: fix tty line-status reporting

commit 5d1678a3 upstream.

Fix handling of TTY error flags, which are not bitmasks and must
specifically not be ORed together as this prevents the line discipline
from recognising them.

Also insert null characters when reporting overrun errors as these are
not associated with the received character.

Fixes: 1da177e4 ("Linux-2.6.12-rc2")
Signed-off-by: Johan Hovold <johan@kernel.org>
Signed-off-by: Jiri Slaby <jslaby@suse.cz>