Do not lock driver mutex in the global LED panel brightness sysfs
accessors brightness_show() and brightness_store().

The mutex locking is unnecessary here. The I2C transfers are guarded by
I2C core locking mechanism, and the LED commands itself do not interfere
with other commands.

Fixes: 089381b2 ("leds: initial support for Turris Omnia LEDs")
Signed-off-by: Marek Behún <kabel@kernel.org>
Reviewed-by: Lee Jones <lee@kernel.org>
Link: https://lore.kernel.org/r/20230802160748.11208-2-kabel@kernel.orgSigned-off-by: Lee Jones <lee@kernel.org>