axi_adxcvr: Avoid warning about unknown synthesis attribute

Comments starting with the word altera are interpreted by the Altera tools
to be synthesis attribute assignments. In this case this is just a generic
comment though which results in a warning that the synthesis attribute is
unknown.

Slightly reword the comment to avoid this. This is not pretty, but better
than having the false positive warning show up in the log.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>

library/altera/axi_adxcvr/axi_adxcvr_up.v

@@ -134,11 +134,11 @@ module axi_adxcvr_up #(
     end
   end
 
-  // altera specific
+  // Specific to Altera
 
   assign up_rparam_s[31:24] = 8'd0;
 
-  // xilinx specific
+  // Specific to Xilinx
 
   assign up_rparam_s[23:16] = 8'd0;