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

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

Since patch "i2c / ACPI: Use 0 to indicate that device does not have
interrupt assigned" [1], 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.

[1] http://lkml.kernel.org/g/<1430908148-201129-3-git-send-email-mika.westerberg@linux.intel.com>

Signed-off-by: Octavian Purdila <octavian.purdila@intel.com>
Reviewed-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>