Staging: iio: impedance-analyzer: ad5933: fix wrong comments

according to datasheet complete control register is of 2 bytes.
http://www.analog.com/media/en/technical-documentation/data-sheets/AD5933.pdf

Signed-off-by: Nizam Haider <nijam.h@hcl.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>