staging:iio:adc:ad7152: Update sample rate, conversion time, digital filter handling

Rename attribute, use sampling_frequency instead.
Attribute now accepts values in Hz.
Delay readout accordingly.

Signed-off-by: Michael Hennerich <michael.hennerich@analog.com>
Signed-off-by: Jonathan Cameron <jic23@cam.ac.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>