leds-lp5523: minor code style fixes

(a) use LP5523_ENABLE rather than magic number 0x40
(b) use min_t() in lp5523_mux_parse()
(c) skip while loop and just return if invalid command

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