serial: 8250_omap: clear rx_running on zero-length DMA completes

On AM33xx RX DMA only triggers when the FIFO reaches the
configured threshold (typically 48 bytes). For smaller bursts
no DMA request is issued and the FIFO is drained by RX timeout.

In this case __dma_rx_do_complete() can legitimately see count == 0.

The current code exits early in this case and does not clear
dma->rx_running, leaving the DMA state inconsistent. This can
prevent RX DMA from restarting and may cause
omap_8250_rx_dma_flush() to fail, marking DMA as broken.

Fix this by clearing dma->rx_running once the DMA transfer has
completed or been terminated, even if no data was transferred.

Fixes: a5fd894 ("serial: 8250: 8250_omap.c: Clear DMA RX running status only after DMA termination is done")
Cc: stable <stable@kernel.org>
Signed-off-by: Matthias Feser <mfe@KBSgmbhfr.onmicrosoft.com>
Reviewed-by: Moteen Shah <m-shah@ti.com>
Link: https://patch.msgid.link/BE3P281MB55155F2F5795E411F5A65282EE0B2@BE3P281MB5515.DEUP281.PROD.OUTLOOK.COM
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Author: Matthias Feser

drivers/tty/serial/8250/8250_omap.c

@@ -944,11 +944,12 @@ static void __dma_rx_do_complete(struct uart_8250_port *p)
 				dev_err(p->port.dev, "teardown incomplete\n");
 		}
 	}
+
+	dma->rx_running = 0;
 	if (!count)
 		goto out;
 	ret = tty_insert_flip_string(tty_port, dma->rx_buf, count);
 
-	dma->rx_running = 0;
 	p->port.icount.rx += ret;
 	p->port.icount.buf_overrun += count - ret;
 out: