ftdi swd: disable SWD output pin during input

ftdi swd: disable SWD output pin during input

 * Disables the data output pin while SWD is reading, so that a simple FTDI
   SWD interface can be made by connecting TCK to SWD_CLK and TDI+TDO directly
   to SWDIO. Enabled by setting SWDIO_OE to 0.

Change-Id: I7d3b71cf3f4eea163cb320aff69ed95d219190bd
Signed-off-by: Patrick Stewart <patstew@gmail.com>
Signed-off-by: Roger Lendenmann <roger.lendenmann@intel.com>
Reviewed-on: http://openocd.zylin.com/2274
Reviewed-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>
Tested-by: jenkins