led: Update led_get_by_label()

led: Update led_get_by_label()

During led_init() execution, led_get_label() returns either the label
property (which is an obsolete property [1]) or the LED's node name.
It can't be the function name as dev parameter is NULL.

Later, during led_post_bind() execution, for the same LED, the attributed
label by led_get_label() can be the function name, as led_get_label()
dev's parameter is set.

During call sequence led_boot_on() => led_boot_get() => led_get_by_label()
with label given in parameter (priv->boot_led_label which is either the
label or node's name set previously in led_init()) can be different to
to uc_plat->label and returns -ENODEV.

Update led_get_by_label() to allow to retrieve LED also by its node name.

[1] https://www.kernel.org/doc/Documentation/devicetree/bindings/leds/common.yaml

Signed-off-by: Patrice Chotard <patrice.chotard@foss.st.com>
Cc: Yegor Yefremov <yegorslists@googlemail.com>