In rfsim, received packets are dynamically allocated and freed when no longer
needed, which is dependent on the channel model, packet timestamp and packet
size (number of samples carried).

This commit fixes the packet age check for channels with nonzero channel offset
(most notalby NTN channels).

Also present:
 - extract the NTN satellite position update code to a separate function and
   call it once per trx_read call instead of once per rx beam.