modem: backend: uart_async: Extend "closed" requisites

The current implementation only waits for the RX disabled event
to determine if the UART is closed. It should wait for all RX
buffers to be released, and the TX to be done as well.

We now also call uart_tx_abort() when closing the pipe.

Signed-off-by: Bjarki Arge Andreasen <bjarki@arge-andreasen.me>