[rf] Adding rffe_set_gain() and rffe_get_gain() to get/set computed gain

[rf] Adding rffe_set_gain() and rffe_get_gain() to get/set computed gain

rffe_compute_gain() is the new name for rffe_set_gain(). I needed to change
this, to solve the name collision with the rffe_set_gain() function, which
actually sets the absolute gain.

rffe_get_gain() will now read the absolute gain which has been computed by
rffe_compute_gain() or set by rffe_set_gain().