iio: trigger: Fix platform_get_irq's error checking

The platform_get_irq() function returns negative if an error occurs.
zero or positive number on success. platform_get_irq() error checking
for zero is not correct.

Signed-off-by: Arvind Yadav <arvind.yadav.cs@gmail.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>