use correct type in call to bladerf_get_frequency

Building against ubuntu bionic (amd64) and current bladeRF sources (4bcdd6ec)
resulted in the following error:

```
    sdr_bladerf.c: In function 'show_config':
    sdr_bladerf.c:116:76: error: passing argument 3 of 'bladerf_get_frequency' from incompatible pointer type [-Werror=incompatible-pointer-types]
             (status = bladerf_get_frequency(BladeRF.device, BLADERF_MODULE_RX, &freq)) < 0 ||
                                                                                ^
    In file included from sdr_bladerf.c:23:0:
    /usr/include/libbladeRF.h:1117:15: note: expected 'bladerf_frequency * {aka long unsigned int *}' but argument is of type 'unsigned int *'
     int CALL_CONV bladerf_get_frequency(struct bladerf *dev,
^~~~~~~~~~~~~~~~~~~~~
```

This commit uses the correct type (`bladerf_frequency`) for the `freq`
variable.

sdr_bladerf.c

@@ -102,7 +102,7 @@ static void show_config()
  int status;
 
  unsigned rate;
- unsigned freq;
+ bladerf_frequency freq;
  bladerf_lpf_mode lpf_mode;
  unsigned lpf_bw;
  bladerf_lna_gain lna_gain;