drivers: serial: nrf_uarte: Fix async/int TX api in inactive state

Currently zephyr has no means to control access to the uart
driver when it is inactive, e.g. shell is not aware of uart
being in idle state and calls asynchronous api. Add early
return to TX starting procedure if device is idle.

Signed-off-by: Krzysztof Chruscinski <krzysztof.chruscinski@nordicsemi.no>