rtc: ds1390: fix ds1390_get_reg return value

spi_write_then_read puts in rx_buf the received data starting from
the first byte of the rx_buf
Signed-off-by: Ivan Grimaldi <grimaldi.ivan@gmail.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>

drivers/rtc/rtc-ds1390.c

@@ -62,7 +62,7 @@ static int ds1390_get_reg(struct device *dev, unsigned char address,
 	if (status != 0)
 		return status;
 
-	*data = chip->txrx_buf[1];
+	*data = chip->txrx_buf[0];
 
 	return 0;
 }