ftdi: Report an error if no ftdi_vid_pid is specified

ftdi: Report an error if no ftdi_vid_pid is specified

By default the list of VID/PID is empty so if ftdi_vid_pid is not called
then no matches are attempted. The only message is at -d3:

	Command 'init' failed with error code -100"

Check for this condition explicitly to make life easier for people
configuring adapters.

Change-Id: If0f93370c9e9ddc9700aae7c346c1c6dd319152e
Signed-off-by: Leonard Crestez <leonard.crestez@nxp.com>
Reviewed-on: http://openocd.zylin.com/5440
Tested-by: jenkins
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>