leds: bcm6328: fix signal source assignment for leds 4 to 7

Each nibble represents 4 LEDs, and in case of the higher register, bit 0
represents LED 4, so we need to use modulus for the LED number as well.

Fixes: fd7b025a

 ("leds: add BCM6328 LED driver")
Signed-off-by: Jonas Gorski <jonas.gorski@gmail.com>
Acked-by: Álvaro Fernández Rojas <noltari@gmail.com>
Signed-off-by: Jacek Anaszewski <jacek.anaszewski@gmail.com>