[si_trx] record frequency in si_frequency_configuration struct

firmware/inc/si_trx.h

@@ -28,6 +28,7 @@
 #include "samd20.h"
 
 struct si_frequency_configuration {
+  uint32_t frequency;
   uint8_t outdiv, band, nprescaler;
   uint32_t m; uint16_t n;
   float lsb_tuning_resolution;

firmware/src/si_trx.c

@@ -435,6 +435,8 @@ static void si_trx_set_tx_pa_duty_cycle(uint8_t pa_duty_cycle)
 void si_trx_get_frequency_configuration(struct si_frequency_configuration* config,
                                         uint32_t frequency)
 {
+  config->frequency = frequency;
+
   /* Higher frequency resolution, but also higher power (~+200µA) */
   config->nprescaler = 2;