samples: reel_board: Change a DTS-derived label to the alias one

Change the label GPIO_KEYS_BUTTON_0_GPIO_CONTROLLER that is generated
for the DT node directly to the corresponding one generated through
the "sw0" alias (i.e. SW0_GPIO_CONTROLLER) to make it consistent with
other DT related labels used in this code (SW0_* and LED?_*).

Signed-off-by: Andrzej Głąbek <andrzej.glabek@nordicsemi.no>