Make Rx synchronous

As were suggested in commentaries about this [post](https://www.reddit.com/r/FPGA/comments/1kwgh6v/show_hn_quickrs232_a_lightweight_synthesizable/) 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
```