iio: adc: at91_adc: fix possible memory leak in at91_adc_allocate_trigger()

In the Linux kernel, the following vulnerability has been resolved: iio: adc: at91_adc: fix possible memory leak in at91_adc_allocate_trigger() If iio_trigger_register() returns error, it should call iio_trigger_free() to give up the reference that hold in iio_trigger_alloc(), so that it can call iio_trig_release() to free memory when the refcount hit to 0.