jtag/drivers/libusb0_common: fix FTBFS (libusb_device_descriptor error)

This struct and libusb_get_device_descriptor() method are not present
in libusb-0.1 API, so when libusb-1.0 is unavailable, this code breaks
the build. Fix by using the appropriate struct (which is apparently
filled automatically on device initialisation).

While at it, change return values for consistency with the callers.

Change-Id: I7d85ab9a70401a155a65122397008ae4d81382fe
Signed-off-by: Paul Fertser <fercerpav@gmail.com>
Reviewed-on: http://openocd.zylin.com/2252
Tested-by: jenkins
Reviewed-by: Austin Phillips <austin_phillips@hotmail.com>
Reviewed-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>