phy: use fftshift'ed buffer for txdataF
So far txdataF buffer has DC RE as the first element. This was prefered most likely to avoid memcpy before iFFT but comes with a small complexity in RE mapping functions. Instead, we use a flat buffer that holds freq domain data starting from first negative SC. After resource mapping is done on all channels, each OFDM symbol is shifted before iFFT so that the buffer start with DC RE. This reduces complexity in mapping functions and later on when passing beam IDs to 7.2 split RU especially when a beam is associated to interleaved RB and/or REs. There is already a memcpy in place to copy txdataF from gNB stuct to RU struct. So this change does not introduce new memcpy.
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