drivers/cmsis_dap_usb: restructure init sequence a bit

drivers/cmsis_dap_usb: restructure init sequence a bit

This fixes the issue of improper initialisation sequence and in
particular makes "cmsis_dap_vid_pid" config specification functional.

Not really elegant but it's in line with the ftdi driver and so can be
reworked in a uniform way later when the internal API is changed.

Change-Id: Ief9fc64ad0ac24e1c66727153f383e4f30a830c7
Signed-off-by: Paul Fertser <fercerpav@gmail.com>
Reviewed-on: http://openocd.zylin.com/2192
Reviewed-by: Andrey Yurovsky <yurovsky@gmail.com>
Tested-by: jenkins
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>