leds:pca963x: Always initialize MODE2 register

PCA9632 defaults to open-drain
PCA9633/4/5 defaults to totem-pole

the driver assumed that totem-pole default and didn't actively set
the value; the MODE2 register is now written if platform
data indicating the mode is given

Signed-off-by: Peter Meerwald <pmeerw@pmeerw.net>
Signed-off-by: Bryan Wu <cooloney@gmail.com>