usrp: try to bring back sanity to pre- and post-processing of FFT data
usrp: try to bring back sanity to pre- and post-processing of FFT data

The pre- and post-processing of FFT data was tweaked based on results
obtained from a mis-configured measurement setup, in an only partially
successful attempt to right the wrong data.

- usrp/fft.c (fft_complex): square the magnitude on output but don't
  square the scaling
- usrp/fft.c (fft_real): square the magnitude on input (does this make
  sense ?)
- usrp/fft.c (fft_real): scale the output with the square root of the
  FFT size (dubious)

cfriedt/ben-wpan,cfriedt/ben-wpan,cfriedt/ben-wpan,cfriedt/ben-wpan,cfriedt/ben-wpan,cfriedt/ben-wpan