leds: lp8788: Fix updating scale configuration bits

We need to do left shift (cfg->num + LP8788_ISINK_SCALE_OFFSET) bits for
updating scale configuration.

Signed-off-by: Axel Lin <axel.lin@gmail.com>
Acked-by: Milo(Woogyom) Kim <milo.kim@ti.com>
Tested-by: Milo(Woogyom) Kim <milo.kim@ti.com>
Signed-off-by: Bryan Wu <bryan.wu@canonical.com>