Fix bug in vfo_comp calculation in Hamlib

I tried to control my "FUNcube Dongle Pro" SDR with Hamlib. At first it
worked fine, but when I tried to correct the frequency divergence with
the vfo_comp parameter, I found a bug in the software.

Example:
$ ./rigctl -m 2513 -C vfo_comp=0.01
  Rig command: F 100000000
  Rig command: f
  Frequency: 102010000

The correction must toke place when setting the frequency and when it
is read back. But one time the correction must work in on direction,
the other time quite opposite.

I found the code in the file rig.c and could see, that both times the
same calculation is used.

The simplest way to correct this is to go to the “rig_get_freq(“
subroutine and change it in the line 1188 from “+=” to “-=” , even that
is mathematically not correct.

Result:
$ ./rigctl -m 2513 -C vfo_comp=0.01
  Rig command: F 100000000
  Rig command: f
  Frequency: 99990000

$ ./rigctl -m 2513 -C vfo_comp=0.000022
  Rig command: F 100000000
  Rig command: f
  Frequency: 99999999

You can see, that the result is quite good for small divergences of
some ppm, but there is still an error for bigger divergences. So I
changed the line 1188 to the form I hope it is the mathematically
correct one:

*freq = (freq_t)(*freq/(1.0+(double)rig->state.vfo_comp));

Result:
$ ./rigctl -m 2513 -C vfo_comp=0.01
  Rig command: F 100000000
  Rig command: f
  Frequency: 100000000

$ ./rigctl -m 2513 -C vfo_comp=0.000022
  Rig command: F 100000000
  Rig command: f
  Frequency: 100000000

Now the result looks good for small and for bigger divergences.

Best regards
Dieter Röver (DK6OV)

src/rig.c

@@ -1183,7 +1183,7 @@ int HAMLIB_API rig_get_freq(RIG *rig, vfo_t vfo, freq_t *freq)
     /* VFO compensation */
     if (rig->state.vfo_comp != 0.0)
     {
-        *freq += (freq_t)(rig->state.vfo_comp * (*freq));
+        *freq = (freq_t)(*freq/(1.0+(double)rig->state.vfo_comp));
     }
 
     if (retcode == RIG_OK