leds-lp5523: change the return type of lp5523_set_mode()

The return value of this function is not handled any place, so
make it as void type.

And three if-statements are replaced with switch-statements.

Signed-off-by: Milo(Woogyom) Kim <milo.kim@ti.com>
Signed-off-by: Bryan Wu <bryan.wu@canonical.com>