stlink_usb.c: add support for STLink V2-1

stlink_usb.c: add support for STLink V2-1

The new version of STLink (V2-1) has a different USB endpoint layout.
As the PID of the device also changed it's possible to change EPs used
by OpenOCD in runtime. The patch adds three new fields to stlink_usb_handle_s
and assigns right EP numbers in stlink_usb_open().
Parts of the code inspired by and used with consent of Jens Hoffmann.

Change-Id: I93b69fb889f15317e9bf864905f435905db39745
Signed-off-by: Bartosz Bielawski <bartosz.bielawski@gmail.com>
Reviewed-on: http://openocd.zylin.com/2019
Tested-by: jenkins
Reviewed-by: Nemui Trinomius <nemuisan_kawausogasuki@live.jp>
Reviewed-by: Paul Fertser <fercerpav@gmail.com>
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>