tty ldisc: Close/Reopen race prevention should check the proper flag

Commit acfa747b introduced the TTY_HUPPING flag to distinguish
closed TTY from currently closing ones. The test in tty_set_ldisc
still remained pointing at the old flag. This causes pppd to
sometimes lapse into uninterruptible sleep when killed and
restarted.

Signed-off-by: Shachar Shemesh <shachar@liveu.tv>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>