rtc: fix drivers that consider 0 as a valid IRQ in client->irq

Since dab472eb ("i2c / ACPI: Use 0 to indicate that device does not
have interrupt assigned"), 0 is not a valid i2c client irq anymore, so
change all driver's checks accordingly.

The same issue occurs when the device is instantiated via device tree
with no IRQ, or from the i2c sysfs interface, even before the patch
above.

Signed-off-by: Octavian Purdila <octavian.purdila@intel.com>
Reviewed-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>