rtc/max6900: use rtc_valid_tm() to check returning tm

Use rtc_valid_tm() to check returning tm for max6900, it can avoid
returning wrong tm value.
Signed-off-by: Wan ZongShun <mcuos.com@gmail.com>
Acked-by: Dale Farnsworth <dale@farnsworth.org>
Cc: Alessandro Zummo <a.zummo@towertech.it>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

drivers/rtc/rtc-max6900.c

@@ -159,7 +159,7 @@ static int max6900_i2c_read_time(struct i2c_client *client, struct rtc_time *tm)
 		      bcd2bin(regs[MAX6900_REG_CENTURY]) * 100 - 1900;
 	tm->tm_wday = bcd2bin(regs[MAX6900_REG_DW]);
 
-	return 0;
+	return rtc_valid_tm(tm);
 }
 
 static int max6900_i2c_clear_write_protect(struct i2c_client *client)