FIX: SDCMD now delays 8 clock cycles between commands, to give the
  SDCard time to process the last command
FIX: Documentation (picture) updated with the current status
FIX: Testbench adjusted to match the SDWB location of the LSB word of
  the DMA address

DMA hardware testing includes read-only, single block testing.  Further
testing to follow.

- Model updated, so STOP_TRANSMISSION kills any transmission
- Updates made to DMA to keep from locking up
  (It still locks up currently)

Update mdl_sdio.v

Update sdaxil.v

Update sim_sdio.pl

Correct some response codes.

* The R2 response from CMD2 must have "111111" in bits 133:128. This agrees with test in hardware.
* The R1b response from CMD7 must have command_index = 7

Remove verilator error about unused signal

This fixes the error:

```
sh: 1: sdiosim: not found
```

Explanation: Some people don't have the current working directory `.` in their $PATH.