stlink: fix connect under reset issues

We need to make sure that srst is asserted before we attempt to switch into
jtag or swd mode otherwise we receive a error (-9) - invalid device id.

Change-Id: I625166c751cfba8e8a5290f40122bb9afc9dbb39
Signed-off-by: Spencer Oliver <spen@spen-soft.co.uk>
Reviewed-on: http://openocd.zylin.com/1315
Tested-by: jenkins