usb_blaster: Don't unnecessarily go through DR-/IR-Pause

usb_blaster: Don't unnecessarily go through DR-/IR-Pause

There is no need to pass through DR-/IR-Pause after a scan if we want to
go to DR-/IR-Update. We just have to skip the first step of the path to
the end state because we already did that step when shifting the last
bit.

v2:
 - Fix comments as remarked in review of v1

Change-Id: I3c10f02794b2233f63d2150934e2768430873caa
Signed-off-by: Daniel Glöckner <daniel-gl@gmx.net>
Reviewed-on: http://openocd.zylin.com/4245
Tested-by: jenkins
Reviewed-by: Christopher Head <chead@zaber.com>
Reviewed-by: Matthias Welwarsky <matthias@welwarsky.de>