leds: lt3593: Remove unneeded assignment in lt3593_led_probe

As the chip can't be queried for it's current state, the "keep" option
doesn't make sense. The only option in DT should be turn the LED on or
off at probe time. Also 'flags' is never used since introduction.

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Jacek Anaszewski <jacek.anaszewski@gmail.com>