leds-bcm6328: support second hw blinking interval

Right now the driver uses only 3 LED modes:
0: On
1: HW Blinking (Interval 1)
3: Off

However, the controller supports a second HW blinking interval, which results
in 4 possible LED modes:
0: On
1: HW Blinking (Interval 1)
2: HW Blinking (Interval 2)
3: Off

Signed-off-by: Álvaro Fernández Rojas <noltari@gmail.com>
Signed-off-by: Pavel Machek <pavel@ucw.cz>