the uci_stats variable (pointing to per-UE statistics for UCI) is used
in function nr_decode_pucch0() to store information which is important
beyond statistics, e.g., uci_stats->pucch0_thres as a threshold (which
comes from somewhere else)

This commit refactors to use original or intermediate variables to not
mix uci_stats usage with actual processing.

Also, group uci_stats a bit more.