leds: lm3601x: Don't use mutex after it was destroyed

The mutex might still be in use until the devm cleanup callback
devm_led_classdev_flash_release() is called. This only happens some time
after lm3601x_remove() completed.

Fixes: e63a7448

 ("leds: lm3601x: Convert class registration to device managed")
Acked-by: Pavel Machek <pavel@ucw.cz>
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Wolfram Sang <wsa@kernel.org>