stlink: fix vector catch not being cleared

stlink: fix vector catch not being cleared

Seems after a reset the stlink is not clearing the vector catch (VC_CORERESET)
in the Debug Control Register.

This has the side effect if the user presses an external reset the core will
halt, this patch fixes that.

Change-Id: Ic3b2c3991b79cacbbd901c02b79613c2e204e71f
Signed-off-by: Spencer Oliver <spen@spen-soft.co.uk>
Reviewed-on: http://openocd.zylin.com/905
Tested-by: jenkins
Reviewed-by: Freddie Chopin <freddie.chopin@gmail.com>