stlink: dequeue CSW write only if it doesn't change csw_default

stlink: dequeue CSW write only if it doesn't change csw_default

The stlink commands for buffer read/write carry the associated CSW
value that has to be used. We can dequeue any CSW write request
and add the CSW in the following buffer read/write.

In preparation to next patch that uses stlink commands misc-rw
(commands that don't handle CSW value), let's dequeue only those
CSW write that don't change csw_default.

Keep a local cache of last csw_default.
Tag the queued CSW writes that change csw_default.
Dequeue only the un-tagged CSW writes.

On buffer read/write commands, limiting the dequeued CSW write
surely adds a performance penalty. But csw_default is not changed
often so the penalty is not significant.

Change-Id: I538d257fe3c434fc97587846d759951384327f02
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: https://review.openocd.org/c/openocd/+/6606
Tested-by: jenkins
Reviewed-by: Tarek BOCHKATI <tarek.bouchkati@gmail.com>