staging:iio:meter:ade7758: Fix list and set of available sample frequencies.

Fix list of available sample frequencies.
Fix ade7758_write_frequency().

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