rtc-m48t59 driver NO_IRQ mode fixup

Since irq in m48t59_private struct is defined as 'unsigned int', which will
make the following if sentence to be never true:

         if (m48t59->irq < 0)
                  m48t59->irq = NO_IRQ;

And thus it will make the m48t59_rtc_probe() is failed when the driver is
working in a no irq mode:
Signed-off-by: NMark Zhan <rongkai.zhan@windriver.com>
Acked-by: NAlessandro 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-m48t59.c

@@ -35,7 +35,7 @@
 struct m48t59_private {
 	void __iomem *ioaddr;
 	unsigned int size; /* iomem size */
-	unsigned int irq;
+	int irq;
 	struct rtc_device *rtc;
 	spinlock_t lock; /* serialize the NVRAM and RTC access */
 };