cmsis-dap: refactor HID PID/VID check loop

In preparation for adding serial number support.

Change-Id: I3c9fb411b79d54a4d2de067039255436ba6708c7
Signed-off-by: Spencer Oliver <spen@spen-soft.co.uk>
Reviewed-on: http://openocd.zylin.com/2328
Tested-by: jenkins
Reviewed-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>
cmsis-dap: add serial number support

Change-Id: I66926d1013e2b3a43ce0d18d3599771428706b6a
Signed-off-by: Spencer Oliver <spen@spen-soft.co.uk>
Reviewed-on: http://openocd.zylin.com/2329
Tested-by: jenkins
Reviewed-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>