[PATCH v2] leds: Add mutex protection in brightness_show()

Initially the claim about no need for lock in brightness_show() was valid as the function was just returning unchanged LED brightness. After the addition of led_update_brightness() this is no longer true, as the function can change the brightness if a LED class driver implements brightness_get

[PATCH v2] leds: Add mutex protection in brightness_show()

Initially the claim about no need for lock in brightness_show() was valid as the function was just returning unchanged LED brightness. After the addition of led_update_brightness() this is no longer true, as the function can change the brightness if a LED class driver implements brightness_get