drivers: serial: pipe: init device at compile time and check if ready
drivers: serial: pipe: init device at compile time and check if ready

Initialize UART pipe device at compile time, allowing to constify the
device pointer. Also fix device ready check (was checking for NULL, an
always true condition when using DEVICE_DT_GET).

Signed-off-by: Gerard Marull-Paretas <bedb45f43a286e49f4dcda039a436b216ae61ef3@nordicsemi.no>

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