iio: adc: ti_am335x_adc: remove omitted iio_kfifo_free()

When the conversion was done to use devm_iio_kfifo_allocate(), a call to
iio_kfifo_free() was omitted (to be removed).
This change removes it.

Fixes: 3c530805

 ("iio: adc: ti_am335x_adc: alloc kfifo & IRQ via devm_ functions")
Signed-off-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Link: https://lore.kernel.org/r/20201203072650.24128-1-alexandru.ardelean@analog.com
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>