tty: n_smux: Prevent TTY Line Discipline unload during flush

Hold mutex lock while flushing TTY to avoid race condition with
unloading the line discipline.

Change-Id: I2327be9bd39119d14c3e607e8ace201d51755e08
Signed-off-by: Eric Holmberg <eholmber@codeaurora.org>
1 file changed