cmsis-dap: prevent hidapi to search again for the adapter

cmsis-dap: prevent hidapi to search again for the adapter

The code in cmsis_dap_usb_open() already searches for the right
HID device that corresponds to the adapter. By calling hid_open()
it asks hidapi to re-search the adapter again based on the VID:PID
and the serial string of the adapter it has just found!

Apart from being a run-time overhead, this has an additional
drawback; there are USB adapters built as composite USB devices
that, beside the cmsis-dap HID interface, have other HID
interfaces for other purposes.
A typical example is the NXP LPC-Link2, that over the 5 interfaces
	0) cmsis-dap (HID)
	1) VCOM-CDC
	2) VCOM-CDC
	3) LPCSIO (HID)
	4) LPC-LINK2 DATA PORT (HID)
has 3 of them of HID class.
The code in cmsis_dap_usb_open() could select the right interface
but then cannot propagate this information to hid_open().

Replace the call to hid_open() with hid_open_path(), passing as
parameter the "unique" path of the HID device already found.

Checking in hidapi source code, the implementation of hid_open()
consists in enumerating the HID devices, scan for the first one
matching VID:PID and serial number, and opening it by calling
hid_open_path(). This analysis highlights that using directly
hid_open_path() should not introduce any regression.

While applying these changes, move hid_init() before enumerating
the HID devices. This has no real consequences because the HID API
is marked as optional but, logically, it should be called before
any other HID API.

Change-Id: I77ec01dca64223ec597f21f188f363366d0049c6
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Suggested-by: Masatoshi Tateishi <tateishim3@gmail.com>
Reviewed-on: http://openocd.zylin.com/5731
Tested-by: jenkins