leds-pca963x: Fix device tree parsing

A malformed device tree could lead into a segmentation fault if the reg
value of a led is bigger than the number of leds.

A valid device tree could have only information about the last led of the
chip. Fix the device tree parsing to handle those cases.

Signed-off-by: Ricardo Ribalda Delgado <ricardo.ribalda@gmail.com>
Signed-off-by: Bryan Wu <cooloney@gmail.com>