vsllink: improve the performance when using swd

Versaloon firmware has been updated for reporting
errors if fail on swd transactions.

Change-Id: I49ac0ad034cc9ad83cc4e43953579811d1243063
Signed-off-by: Simon Qian <openocd@versaloon.com>
Reviewed-on: http://openocd.zylin.com/2302
Reviewed-by: Fatih Aşıcı <fatih.asici@gmail.com>
Tested-by: jenkins
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>