configure the SWD frequency when setting adapter speed in SWD mode for versaloon

Change-Id: I99cdc11ba1442e4c9efaa0f1de8e7089ec725e14
Signed-off-by: Simon Qian <openocd@versaloon.com>
Reviewed-on: http://openocd.zylin.com/2608


Tested-by: jenkins
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>