leds: trigger: Fix error path to not unlock the unlocked mutex

ttyname is allocated before the mutex is taken, so it must not be
unlocked in the error path.

Fixes: fd4a641a ("leds: trigger: implement a tty trigger")
Reported-by: Pavel Machek <pavel@ucw.cz>
Acked-by: Pavel Machek <pavel@ucw.cz>
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Link: https://lore.kernel.org/r/20210219133307.4840-2-u.kleine-koenig@pengutronix.de


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>