leds: clevo-mail: Make probe function __init

One of the benefits of platform_driver_probe() is that you can make
the probe function __init.

Signed-off-by: Jean Delvare <jdelvare@suse.de>
Cc: Bryan Wu <cooloney@gmail.com>
Cc: Richard Purdie <rpurdie@rpsys.net>
Signed-off-by: Bryan Wu <cooloney@gmail.com>