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.