Simplify and explain csr_imm_en

rtl/serv_decode.v

@@ -144,7 +144,7 @@ module serv_decode
 
    assign o_csr_source = funct3[1:0];
    assign o_csr_d_sel = funct3[2];
-   assign o_csr_imm_en = csr_op & o_csr_d_sel;
+   assign o_csr_imm_en = opcode[4] & opcode[2] & funct3[2];
 
    assign o_csr_addr = (op26 & !op20) ? CSR_MSCRATCH :
 		       (op26 & !op21) ? CSR_MEPC :

rtl/serv_immdec.v

@@ -30,6 +30,7 @@ module serv_immdec
 
    always @(posedge i_clk) begin
       if (i_wb_en) begin
+	 /* CSR immediates are always zero-extended, hence clear the signbit */
 	 signbit     <= i_wb_rdt[31] & !i_csr_imm_en;
 	 imm19_12_20 <= {i_wb_rdt[19:12],i_wb_rdt[20]};
 	 imm7        <= i_wb_rdt[7];