jtag ftdi: sample TDO on falling edge of TCK

jtag ftdi: sample TDO on falling edge of TCK

Due to signal propagation delays, sampling TDO on rising TCK can become
quite peculiar at fast TCK rates. However, FTDI chips offer a possiblity
to sample TDO on falling edge. With this change, stable operation can be
achieved at 30MHz clock even over 10cm ribbon cable.

Change-Id: Icaf240535dae15512e3c60a944e22a5fbc1b0b06
Signed-off-by: Matthias Welwarsky <matthias@welwarsky.de>
Reviewed-on: http://openocd.zylin.com/3180
Tested-by: jenkins
Reviewed-by: Paul Fertser <fercerpav@gmail.com>