iio: accel: sca3000_core: implemented IIO_CHAN_INFO_SAMP_FREQ

Moved functionality from IIO_DEV_ATTR_SAMP_FREQ attribute into
IIO_CHAN_INFO_SAMP_FREQ handlers. Added sca3000_write_raw() to allow
writing the element as well.

Signed-off-by: Ico Doornekamp <ico@pruts.nl>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>