iio: accel: adxl380: Store sampling frequency index in odr struct member

iio: accel: adxl380: Store sampling frequency index in odr struct member

The ADXL380 driver assumes that acceleration samples are always retrieved
via the high-performance DSM signal path; as a result, the sampling
frequency value depends exclusively on the decimation filter settings in
the TRIG_CFG register.
In preparation for adding support for sampling frequency values that rely
on the low-power SAR signal path (on which the decimation filters are not
supported), use the (currently unused) 'odr' member of struct adxl380_state
to store the sampling frequency value, and when userspace requests the
current frequency value, retrieve it from the struct instead of calculating
it from the decimation filter settings.

Signed-off-by: Francesco Lavra <flavra@baylibre.com>
Reviewed-by: Nuno Sá <nuno.sa@analog.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>