iio: adc: ad7768-1: remove switch to one-shot mode

iio: adc: ad7768-1: remove switch to one-shot mode

wideband low ripple FIR Filter is not available in one-shot mode. In
order to make direct reads work for all filter options, remove the
switch for one-shot mode and guarantee device is always in continuous
conversion mode.

Fixes: fb1d3b24ebf5 ("iio: adc: ad7768-1: add filter type and oversampling ratio attributes")
Signed-off-by: Jonathan Santos <Jonathan.Santos@analog.com>
Reviewed-by: David Lechner <dlechner@baylibre.com>
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>