SWD_Sequence() off-by-one in reference implementation

[flit]

There is a reported off-by-one error in value returned by the implementation of SWD_Sequence() in SW_DP.c.

Description of patch with fix:

The original SWD sequence has an off-by-one error that causes
all returned values to be shifted. This isn't an issue for most
software, since they tend to use `SWD_Sequence()` as a write-only
primitive and don't care about the return value.

The Black Magic Debug Project uses this for additional checks,
which works around this bug if it's detected. For details, see:
https://github.com/blackmagic-debug/blackmagic/blob/31d30ada18862cdf44ae7ed798325cb7366d05e0/src/platforms/hosted/dap_command.c#L428-L458

This bug is present in reference code from ARM. If this file is
updated to a newer build, please ensure this fix is carried
forward, or ensure the newer version no longer has this bug.

Signed-off-by: Sean Cross <sean@xobs.io>

Link to above: devanlai/dap42@d67619b

(Please note that I have not had time to verify this issue or the patch, or to create a pull request, but I wanted to create an issue so it doesn't get lost.)

[RobertRostohar]

It does not look to me that there is a bug in the original SWD sequence code. I have tried a few test patterns and the original code (with the line val >>= k;) seems to work as expected. May I ask for some tests patterns that would show the issue.