[FHI72] Fix the PRACH FFT size
ru_config->fftSize is used for the C-plane Section Type 3 which is
dedicated for PRACH and mixed numerology. Previously, we assumed that
PRACH FFT size is equivalent to PxSCH FFT size, but should be based on
the PRACH format.
Tests with the following RUs:
- [x] Benetel v1.2.2 & v1.4.1
- [x] VVDN Gen 3
- [x] Liteon 02.00.10
- [x] Metanoia v2.2.8 (Jura)