fpga: rfnoc: fft: Fix tready on magnitude path

The tready signal that was used when EN_MAGNITUDE equals 0 was not
correct. This commit allows EN_MAGNITUDE to be disabled.

fpga/usrp3/lib/rfnoc/blocks/rfnoc_block_fft/fft_post_processing.sv

@@ -280,7 +280,7 @@ module fft_post_processing #(
     assign mag_out_tdata  = '0;
     assign mag_out_tlast  = '0;
     assign mag_out_tvalid = '0;
-    assign s_axis_tready  = '1;
+    assign mag_in_tready  = '1;
   end
 
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