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 <[e-mail]>
Signed-off-by: Alexandre Belloni <[e-mail]>

drivers/rtc/rtc-ds1390.c

···
 	if (status != 0)
 		return status;
 
-	*data = chip->txrx_buf[1];
+	*data = chip->txrx_buf[0];
 
 	return 0;
 }