swd: Remove support for turnaround periods other than 1

swd: Remove support for turnaround periods other than 1

ARM deprecated other trn periods in ADIv5.1 and one cycle is the only
setting that is guaranteed to be implemented, as well as being the reset
value in ADIv5.0.

Thus it makes no sense supporting anything else.

Change-Id: Iffa16bb0ce74788bca88fd3ace8a026148013d00
Signed-off-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>
Reviewed-on: http://openocd.zylin.com/2132
Tested-by: jenkins
Reviewed-by: Paul Fertser <fercerpav@gmail.com>