HID: lenovo: Use brightness_set_blocking callback for setting LEDs brightness

The lenovo_led_brightness_set function may sleep, so we should have the
the led_class_dev's brightness_set_blocking callback point to it, rather
then the regular brightness_set callback.

When toggled through sysfs this is not a problem, but the brightness_set
callback may be called from atomic context when using LED-triggers.

Fixes: bc04b37e ("HID: lenovo: Add ThinkPad 10 Ultrabook Keyboard support")
Reviewed-by: Marek Behún <kabel@kernel.org>
Acked-by: Pavel Machek <pavel@ucw.cz>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>