leds: leds-gpio: Fix the "default-state" property check

Commit a43f2cbb ("leds: leds-gpio: Make use of device property API")
caused a regression by reading the incorrect string for the "default-state"
property.

Fix the property string check to restore the original behaviour.

Fixes: a43f2cbb (leds: leds-gpio: Make use of device property API)
Reported-by: Jean-Michel Hautbois <jean-michel.hautbois@vodalys.com>
Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Acked-by: Bryan Wu <cooloney@gmail.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>