rtc: m41t80: fix SQW dividers override when setting a date

This patch is only relevant for RTC with the SQ_ALT feature which
means the clock output frequency divider is stored in the weekday
register.

Current implementation discards the previous dividers value and clear
them as soon as the time is set.
Signed-off-by: Gary Bisson <gary.bisson@boundarydevices.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>

drivers/rtc/rtc-m41t80.c

@@ -227,6 +227,7 @@ static int m41t80_get_datetime(struct i2c_client *client,
 /* Sets the given date and time to the real time clock. */
 static int m41t80_set_datetime(struct i2c_client *client, struct rtc_time *tm)
 {
+	struct m41t80_data *clientdata = i2c_get_clientdata(client);
 	unsigned char buf[8];
 	int err, flags;
 
@@ -242,6 +243,17 @@ static int m41t80_set_datetime(struct i2c_client *client, struct rtc_time *tm)
 	buf[M41T80_REG_YEAR] = bin2bcd(tm->tm_year - 100);
 	buf[M41T80_REG_WDAY] = tm->tm_wday;
 
+	/* If the square wave output is controlled in the weekday register */
+	if (clientdata->features & M41T80_FEATURE_SQ_ALT) {
+		int val;
+
+		val = i2c_smbus_read_byte_data(client, M41T80_REG_WDAY);
+		if (val < 0)
+			return val;
+
+		buf[M41T80_REG_WDAY] |= (val & 0xf0);
+	}
+
 	err = i2c_smbus_write_i2c_block_data(client, M41T80_REG_SSEC,
 					     sizeof(buf), buf);
 	if (err < 0) {