serial: uart_native_tty: Polling thread fix

Addresses an issue where attempts to transmit data fail.
The identified cause of this failure is the handling of the
`rx_irq_enable` and `tx_irq_enable` flags. The `else if` handling the
`tx_irq_enable` was replaced by an `if` so both flags can be true,
and both functions can be called. This fix was tested with the
`cellular_modem` example successfully.

Signed-off-by: Fabian Kainka <kainka@cognid.de>
diff --git a/drivers/serial/uart_native_tty.c b/drivers/serial/uart_native_tty.c
index dc19f2a..8eb51bf 100644
--- a/drivers/serial/uart_native_tty.c
+++ b/drivers/serial/uart_native_tty.c
@@ -289,9 +289,11 @@
 			} else {
 				k_sleep(K_MSEC(1));
 			}
-		} else if (data->tx_irq_enabled) {
+		}
+		if (data->tx_irq_enabled) {
 			native_tty_uart_irq_handler(dev);
-		} else {
+		}
+		if (data->tx_irq_enabled == false && data->rx_irq_enabled == false) {
 			k_sleep(K_MSEC(10));
 		}
 	}