Fix frequency help texts in HIGH mode

Only span should be shown from 0..720 MHz, start, stop, center, ...
should be 240..960 MHz.

Fixes: https://github.com/erikkaashoek/tinySA/issues/26

ui.c

@@ -946,7 +946,7 @@ menu_invoke(int item)
     if (menu->data <= KM_CW) {      // One of the frequency input keypads
       if (MODE_LOW(setting.mode))
         kp_help_text = VARIANT("0..350MHz",range_text);
-      else if (menu->data <= KM_SPAN)
+      else if (menu->data == KM_SPAN)
         kp_help_text = VARIANT("0..720Mhz",range_text);
       else
         kp_help_text = VARIANT("240..960Mhz",range_text);