Reformat code to properly copy memory if we skip precoding.

precoding_stats timing is not part of ru->feptx_ofdm, but
ru->feptx_prec, so always print it.

Previously, in order to speed up processing if precoding is disabled, we
just copy the pointer from txdataF to txdataF_BF, as opposed to do a
full memcpy() to duplicate the data. The same happens if we cannot have
precoding, i.e., in SISO. This has implications such as memory leaks,
which is why this is not being done. However, the code comment is
intentionally placed there.