USB: keyspan: fix tty line-status reporting

USB: keyspan: fix tty line-status reporting

commit 5d1678a33c731b56e245e888fdae5e88efce0997 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: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Signed-off-by: Johan Hovold <johan@kernel.org>
[bwh: Backported to 3.2:
 - s/\&port->port/tty/
 - Adjust context, indentation]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>