drivers: serial: uart_async_rx: Add buffer release to consuming function

Add buffer releasing to consuming function to allow better buffer
utilization since buffer is available earlier.

Note that releasing in claiming function is still needed as there
are cases when there are empty but completed buffers and those
need to be consumed in claiming phase.

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