iio: adc: ti_am335x_adc: alloc channels via devm_kcalloc()

This change attaches the life-cycle of the channels array to the parent
device object that is attached to the IIO device.
This way we can remove from the cleanup code, the explicit
tiadc_channels_remove() which simply does a kfree() on the channels array.

Signed-off-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>