tty: Don't reenable already enabled ldisc

tty_ldisc_hangup() guarantees the ldisc is enabled (or that there
is no ldisc). Since __tty_hangup() was the only user, re-define
tty_ldisc_enable() in file-scope.

Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --cc drivers/tty/tty_io.c
Simple merge

diff --cc drivers/tty/tty_ldisc.c
Simple merge

diff --cc include/linux/tty.h
Simple merge