drivers: led: bcm6858: Set a default brightness when probing LEDs

When probing the LEDs, a default brightness is set based on settings
from the U-Boot device tree, i.e. the 'default-brightness' property
of the LED nodes. If that property is not present, the default
maximum brightness is set.
This should make sure the LED controller's registers affecting the
brightness are correctly initialized and should give a consistent
behaviour.

Signed-off-by: Paul HENRYS <paul.henrys_ext@softathome.com>