Fix the PRACH FFT size for 7.2 interface

We assumed that PRACH FFT size is equivalent to PxSCH FFT size.

However, the parameter ru_config->fftSize is used for the C-plane Section Type 3
which is dedicated for PRACH and mixed numerology.
Based on the PRACH format, short or long, the PRACH values 256 or 1024
shall be filled.

Co-authored-by: knopp <raymond.knopp@eurecom.fr>

radio/COMMON/common_lib.h

@@ -199,6 +199,8 @@ typedef struct split7_config {
   /*! this is the exponent in 2^X for the FFT size */
   uint16_t fftSize;
 
+  /*! this is the ceil of the exponent in 2^X for the prach FFT size */
+  uint16_t prach_fftSize;
   // M-plane related parameters
   uint16_t dl_k0[5];
   uint16_t ul_k0[5];