tests: uart_mix_fifo_poll: Convert test to use DEVICE_DT_GET
tests: uart_mix_fifo_poll: Convert test to use DEVICE_DT_GET

Move to use DEVICE_DT_GET instead of device_get_binding as we
work on phasing out use of DTS 'label' property.

Signed-off-by: Kumar Gala <383ef5577c6e1178b93f59ec8d0936f76d2a98c4@kernel.org>

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