api: uart: Revert to using bitmasks for errors.
api: uart: Revert to using bitmasks for errors.

Revert to using bitmasks for errors.

Signed-off-by: Mieszko Mierunski <0153720f373f9fb33402fd56a3b748bfb748366c@nordicsemi.no>

galak/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,nashif/zephyr,ldts/zephyr,Vudentz/zephyr,finikorg/zephyr,galak/zephyr,GiulianoFranchetto/zephyr,finikorg/zephyr,explora26/zephyr,nashif/zephyr,finikorg/zephyr,nashif/zephyr,GiulianoFranchetto/zephyr,ldts/zephyr,explora26/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,explora26/zephyr,galak/zephyr,Vudentz/zephyr,explora26/zephyr,galak/zephyr,finikorg/zephyr,Vudentz/zephyr,ldts/zephyr,explora26/zephyr,ldts/zephyr,Vudentz/zephyr,Vudentz/zephyr,finikorg/zephyr,nashif/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,GiulianoFranchetto/zephyr,ldts/zephyr,nashif/zephyr,GiulianoFranchetto/zephyr,galak/zephyr,GiulianoFranchetto/zephyr