Hi,
you should not write registers directly!
There is a way to customize these values:
See here:
http://wiki.analog.com/resources/tools-software/linux-software/fmcomms2_advanced_plugin#gain_control
Determine the proper controls and then update these parameters to your devicetree.
See here:
http://wiki.analog.com/resources/tools-software/linux-drivers/iio-transceiver/ad9361-customization
-Michael