gain_dB_to_code
ret = gain_dB_to_code(st, val, val2, &code);
.gain_dB_to_code = hmc425a_gain_dB_to_code,
.gain_dB_to_code = hmc540s_gain_dB_to_code,
.gain_dB_to_code = adrf5740_gain_dB_to_code,
.gain_dB_to_code = ltc6373_gain_dB_to_code,
int (*gain_dB_to_code)(int gain, int *code);
return st->chip_info->gain_dB_to_code(gain, code);