samples: subsys: mgmt: updatehub: Update sample to use DEVICE_DT_GET_ONE
samples: subsys: mgmt: updatehub: Update sample to use DEVICE_DT_GET_ONE

Update sample to use DEVICE_DT_GET_ONE in order to remove DT_INST
usage.

Signed-off-by: Benjamin Björnsson <8044ef148b03b50e606a03e6e88e2c21cb9a92a8@gmail.com>

galak/zephyr,galak/zephyr,finikorg/zephyr,galak/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,finikorg/zephyr