iio: adc: at91: call input_free_device() on allocated iio_dev
In the Linux kernel, the following vulnerability has been resolved:

iio: adc: at91: call input_free_device() on allocated iio_dev

Current implementation of at91_ts_register() calls input_free_deivce()
on st->ts_input, however, the err label can be reached before the
allocated iio_dev is stored to st->ts_input. Thus call
input_free_device() on input instead of st->ts_input.