Fixes format type warning

The Printf format type was unsigned long, but the variable is uint64_t. 

Scanning dependencies of target hackrf_transfer
[100%] Building C object hackrf-tools/src/CMakeFiles/hackrf_transfer.dir/hackrf_transfer.c.o
/Users/Luke/Downloads/hackrf-2013.06.1/host/hackrf-tools/src/hackrf_transfer.c:624:33: warning: 
      format specifies type 'unsigned long' but the argument has type 'uint64_t'
      (aka 'unsigned long long') [-Wformat]
        printf("call hackrf_set_freq(%lu Hz/%.03f MHz)\n", freq_hz, ...
                                     ~~^                   ~~~~~~~
                                     %llu                           
/Users/Luke/Downloads/hackrf-2013.06.1/host/hackrf-tools/src/hackrf_transfer.c:643:29: warning: 
      format specifies type 'unsigned long' but the argument has type 'uint64_t'
      (aka 'unsigned long long') [-Wformat]
                printf("samples_to_xfer %lu/%lluMio\n", samples_to_xfer...
                                        ~~^             ~~~~~~~~~~~~~~~
                                        %llu 
2 warnings generated.