Handle division by 0 assert gracefully in average channel level calculation

* nb_re_pdsch is directly linked to nb_rb_pdsch, if the latter is 0 the
  former is also 0
* compute average channel level only when number of RE for PDSCH is not 0
  the average channel level will be set to 0 otherwise
* Added a log message to handle this special case, which is consequence
  of other issues in the RX chain
* with this commit, the inconsistency in the RX chain will be handled
  in later in nr_ue_pdsch_procedures with a demodulation failure

other minor changes:

* Improved clarity by using NR_NB_SC_PER_RB instead of hard-coded value 12 in nb of RBs calculation
* cleaned up unused variable and outdated comments