driver/ftdi: skip trst in swd mode

driver/ftdi: skip trst in swd mode

When using the adapter olimex arm-jtag-swd (to convert to SWD a
JTAG-only FTDI adapter), the pin trst on JTAG side is re-used to
control the direction of pin SWDIO on SWD side.
There is a single reset API at adapter driver to assert/deassert
either srst and/or trst. A request to assert/deassert srst can
cause also trst to change value, hanging the SWD communication.

In SWD mode, ignore the value passed to trst.

Change-Id: I5fe1eed851177d405d77ae6079da9642dc1a08f1
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: http://openocd.zylin.com/6006
Tested-by: jenkins