NR UE: fix PDCCH LLR indexing for different search space RB sizes

After !3603 (merged), when multiple search spaces have different CORESET
configurations (different number of RBs), the LLR buffer stride must be
calculated based on the maximum RB size across all search spaces, not the
current search space's RB size.

The LLR buffer is allocated with size based on get_pdcch_max_rbs(), but the
demapping function was using coreset_nbr_rb (current search space) for symbol
stride calculation, causing incorrect LLR extraction when search spaces have
different RB configurations.

This MR tries to fix it.

Closes #1021