target/adi_v5_swd: drop unused `swd_driver` in `swd_queue_dp_write()`

target/adi_v5_swd: drop unused `swd_driver` in `swd_queue_dp_write()`

`struct swd_driver swd` is only used in `swd_queue_dp_write()` in an
assertion triggerring `-Wunused-variable` when compiled with `DNDEBUG`.
Drop it.

Change-Id: Id3283b9e2c36a74cda9fc4afc16da02ac4d62b69
Signed-off-by: Evgeniy Naydanov <evgeniy.naydanov@syntacore.com>
Reviewed-on: https://review.openocd.org/c/openocd/+/8754
Tested-by: jenkins
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>