Some clean up of sdp_station.vhd and fifo fill eop usage

Use c_nof_beamlets_per_data = c_sdp_cep_nof_beamlets_per_longword = 2 dual pol beamlets per 64b data word.

Closes L2SDP-776

requested review from @walle

assigned to @kooistra

merged

mentioned in commit 580f666b