rs485echo: Fix RS485 transmission delays

Before, the delay_rts_before_send delay_rts_after_send fields were not
explicitly initialized. This led to a erroneous transmission behaviour.
Now the RS485 port settings are read before the half duplex
configuration is done. This ensures that the delays are set to 0 or to
a predefined value (device tree).

BCS 746-000908

testcases/rs485echo/rs485echo.c

@@ -259,6 +259,14 @@ setup()
 	}
 
 	if (use_rs485) {
+		// Read current RS484 config
+		if (ioctl(fd, TIOCGRS485, &rs485) != 0)
+		{
+			tst_brkm(TBROK, cleanup,
+				"Unable to get RS485 config via ioctl %s.  Error:%d, %s\n",
+				device_name, errno, strerror(errno));
+		}
+
 		// Enable RS485 half duplex mode
 		rs485.flags = SER_RS485_ENABLED | SER_RS485_RTS_ON_SEND;
 		if (ioctl(fd, TIOCSRS485, &rs485) != 0)