nuc900/rtc: change the waiting for device ready implement

- add an mdelay(1) to the polling loop to cause less frequent access to
  the hardware register.

- change the return value from ENODEV to EPERM if the loop timed out.  I
  think the 'Operation not permitted' description is more suitable for the
  meaning of 'check_rtc_access_enable()' function, it just be used to
  judge rtc access operation is permitted or not.
Signed-off-by: NWan ZongShun <mcuos.com@gmail.com>
Cc: Alessandro Zummo <a.zummo@towertech.it>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

drivers/rtc/rtc-nuc900.c

@@ -85,22 +85,21 @@ static irqreturn_t nuc900_rtc_interrupt(int irq, void *_rtc)
 
 static int *check_rtc_access_enable(struct nuc900_rtc *nuc900_rtc)
 {
-	unsigned int i;
+	unsigned int i, timeout = 0x1000;
 	__raw_writel(INIRRESET, nuc900_rtc->rtc_reg + REG_RTC_INIR);
 
 	mdelay(10);
 
 	__raw_writel(AERPOWERON, nuc900_rtc->rtc_reg + REG_RTC_AER);
 
-	for (i = 0; i < 1000; i++) {
-		if (__raw_readl(nuc900_rtc->rtc_reg + REG_RTC_AER) & AERRWENB)
-			return 0;
-	}
+	while (!(__raw_readl(nuc900_rtc->rtc_reg + REG_RTC_AER) & AERRWENB)
+								&& timeout--)
+		mdelay(1);
 
-	if ((__raw_readl(nuc900_rtc->rtc_reg + REG_RTC_AER) & AERRWENB) == 0x0)
-		return ERR_PTR(-ENODEV);
+	if (!timeout)
+		return ERR_PTR(-EPERM);
 
-	return ERR_PTR(-EPERM);
+	return 0;
 }
 
 static void nuc900_rtc_bcd2bin(unsigned int timereg,