iio: dummy_evgen: modify the return value check for irq_sim_init()

iio: dummy_evgen: modify the return value check for irq_sim_init()

As discussed with Marc Zyngier: irq_sim_init() and its devres variant
should return the base of the allocated interrupt range on success
rather than 0. This will be modified later - first, change the way
users handle the return value of these routines.

Signed-off-by: Bartosz Golaszewski <brgl@bgdev.pl>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>