rtc: rtc-ds1374: fix 'no irq' case handling

On a PowerPC board with ds1374 RTC I'm getting this error while RTC tries
to probe:

rtc-ds1374 0-0068: unable to request IRQ

This happens because I2C probing code (drivers/of/of_i2c.c) is specifying
IRQ0 for 'no irq' case, which is correct.

The driver handles this incorrectly, though. This patch fixes it.

Signed-off-by: Anton Vorontsov <avorontsov@ru.mvista.com>
Cc: David Brownell <david-b@pacbell.net>
Cc: Alessandro Zummo <a.zummo@towertech.it>
Acked-by: Peter Korsgaard <jacmet@sunsite.dk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>