jtag/core: honour SRST timings in SWD mode

jtag/core: honour SRST timings in SWD mode

Since SWD is currently abusing the JTAG command queue for reset and
sleep handling (and all other operations are performed in a different
way), sleep needs to be forced explicitly to ensure correct timings.

Change-Id: I5b0da6cbb7d0560154e4077b261aa6828cefc892
Signed-off-by: Paul Fertser <fercerpav@gmail.com>
Reviewed-on: http://openocd.zylin.com/2591
Tested-by: jenkins
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>