Pucch2 polar rx: fix dynamic range

fixes the overflow issue in the PUCCH2 RX with 4+ RX antennas and 12+
bits (polar code format). The fix scales the polar decoding LLR
computation according to the number of RX antennas. The issue was clear
for 100 MHz 4 RX antennas. Even

  nr_pucchsim -s -5 -q 8 -P 2 -b 12 -R 273 -z4 -n1000

doesn't converge to 0 BLER with increasing SNR. The issue didn't appear
in CI because of the bandwidth and antenna count limitations.

openair1/PHY/NR_TRANSPORT/pucch_rx.c

@@ -1519,7 +1519,7 @@ void nr_decode_pucch2(PHY_VARS_gNB *gNB,
 
             corr_re = ( corr32_re[symb][half_prb>>2][aa]/(2*nc_group_size*4/2)+((int16_t*)(&prod_re[aa]))[0]);
             corr_im = ( corr32_im[symb][half_prb>>2][aa]/(2*nc_group_size*4/2)+((int16_t*)(&prod_im[aa]))[0]);
-            corr_tmp += corr_re*corr_re + corr_im*corr_im;
+            corr_tmp += (corr_re*corr_re + corr_im*corr_im)>>(Prx/2);
             /*
               LOG_D(PHY,"pucch2 half_prb %d cw %d (%d,%d) aa %d: (%d,%d,%d,%d,%d,%d,%d,%d)x(%d,%d,%d,%d,%d,%d,%d,%d) (%d,%d)+(%d,%d)
               = (%d,%d) => %d\n", half_prb,cw,cw&15,cw>>4,aa,