Prepare variable sweep points support

Need only add fixes to waterfall mode

main.c

@@ -980,6 +980,14 @@ bool sweep(bool break_on_operation)
 }
 #endif
 
+void set_sweep_points(uint16_t points){
+  if (points == sweep_points || points > POINTS_COUNT)
+    return;
+
+  sweep_points = points;
+  update_frequencies();
+}
+
 VNA_SHELL_FUNCTION(cmd_scan)
 {
   uint32_t start, stop;
@@ -2695,10 +2703,7 @@ int main(void)
   dacStart(&DACD2, &dac1cfg1);
 #endif
   setupSA();
-  sweep_points = POINTS_COUNT;
+  set_sweep_points(POINTS_COUNT);
-/* initial frequencies */
-  update_frequencies();
-
 
 #ifdef __VNA__
 /*

nanovna.h

@@ -123,6 +123,7 @@ enum stimulus_type {
   ST_START=0, ST_STOP, ST_CENTER, ST_SPAN, ST_CW
 };
 
+void set_sweep_points(uint16_t points);
 void update_frequencies(void);
 void set_sweep_frequency(int type, uint32_t frequency);
 uint32_t get_sweep_frequency(int type);