rtc: mrst: remove set but not used variable 'valid'

Fixes gcc '-Wunused-but-set-variable' warning:

drivers/rtc/rtc-mrst.c: In function 'mrst_procfs':
drivers/rtc/rtc-mrst.c:264:29: warning:
 variable 'valid' set but not used [-Wunused-but-set-variable]
  unsigned char rtc_control, valid;

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>

drivers/rtc/rtc-mrst.c

@@ -261,11 +261,10 @@ static int mrst_rtc_alarm_irq_enable(struct device *dev, unsigned int enabled)
 
 static int mrst_procfs(struct device *dev, struct seq_file *seq)
 {
-	unsigned char	rtc_control, valid;
+	unsigned char	rtc_control;
 
 	spin_lock_irq(&rtc_lock);
 	rtc_control = vrtc_cmos_read(RTC_CONTROL);
-	valid = vrtc_cmos_read(RTC_VALID);
 	spin_unlock_irq(&rtc_lock);
 
 	seq_printf(seq,