Make Rx synchronous

[EvilLord666]

As were suggested in commentaries about this post we should use rxd as follows:

logic [2:0] rxd;

// synchronize to the incoming RX data line using a 3-bit shift register
always_ff @(posedge clk) begin
  rxd <= {rxd[1:0], RX};
end

// use rxd[2] instead of RX when shifting in data bits