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: Mark Zhan <rongkai.zhan@windriver.com>
Acked-by: Alessandro Zummo <a.zummo@towertech.it>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>