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>
diff --git a/drivers/rtc/rtc-ds1390.c b/drivers/rtc/rtc-ds1390.c
index e67bfcb..a4303b4 100644
--- a/drivers/rtc/rtc-ds1390.c
+++ b/drivers/rtc/rtc-ds1390.c
@@ -62,7 +62,7 @@
 	if (status != 0)
 		return status;
 
-	*data = chip->txrx_buf[1];
+	*data = chip->txrx_buf[0];
 
 	return 0;
 }