drivers/bitbang: add support for SWD multidrop

drivers/bitbang: add support for SWD multidrop

Ignore ack received after DP_TARGETSEL write to prevent false error.

This change also fixes a bug:
Received ACK FAULT or JUNK value were incorrectly stored to queued_retval
and later used as bitbang_swd_run_queue() error return.

Use LOG_ERROR for parity mismatch.

Change-Id: I5ff1f658f221af78d8bbec8416a7a0fc64ba2550
Signed-off-by: Tomas Vanek <vanekt@fbl.cz>
Reviewed-on: https://review.openocd.org/c/openocd/+/6700
Tested-by: jenkins
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>