iio: light: hid-sensor-prox: Fix memory leak in probe()

'channels' is allocated via kmemdup and it is never freed.

Use 'indio_dev->channels' directly instead, so that we avoid such
memory leak problem.

Reported-by: Alexey Khoroshilov <khoroshilov@ispras.ru>
Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Reviewed-by: Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>