iio: adc: ad7124: Enable internal reference

When the internal reference was selected by a channel it was not enabled.
This patch fixes that and enables it.

Fixes: b3af341b ("iio: adc: Add ad7124 support")
Signed-off-by: Mircea Caprioru <mircea.caprioru@analog.com>
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>