rtc: ds1390: fix number of bytes read from RTC

The spi_write_then_read() reads 8 bytes starting from
DS1390_REG_SECONDS (== 0x01), so the last byte read would already
be part of the alarm (Tenths and Hundredths of Seconds) feature.

However 7 bytes are engouh -- seconds (0x01), minutes (0x02), hours (0x03),
day (0x04), date (0x05), month/century (0x06) and year (0x07).

Signed-off-by: Andreas Gabriel-Platschek <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Alexandre Belloni <[e-mail]>

drivers/rtc/rtc-ds1390.c

···
 	chip->txrx_buf[0] = DS1390_REG_SECONDS;
 
 	/* do the i/o */
-	status = spi_write_then_read(spi, chip->txrx_buf, 1, chip->txrx_buf, 8);
+	status = spi_write_then_read(spi, chip->txrx_buf, 1, chip->txrx_buf, 7);
 	if (status != 0)
 		return status;