leds: gpio: fix and simplify reading property "label"

Checking for the presence of the property first isn't strictly needed
as we can react on the return code of fwnode_property_read_string.
Also, even if the presence of a property "label" was checked,
reading a string value for it theoretically still can fail and
this case isn't handled.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: Jacek Anaszewski <j.anaszewski@samsung.com>