stlink: enable connect under reset

stlink: enable connect under reset

Currently if the target supports srst_nogate we wait until target assert_reset
until we get a chance to assert the srst.
However sometimes we will not get this far if the target has already failed
the initial scan.

This has been tested on stm32.

Change-Id: I2c4486942a011534d3e2044788563669bf457b60
Signed-off-by: Spencer Oliver <spen@spen-soft.co.uk>
Reviewed-on: http://openocd.zylin.com/972
Reviewed-by: Paul Fertser <fercerpav@gmail.com>
Tested-by: jenkins
Reviewed-by: Freddie Chopin <freddie.chopin@gmail.com>