tcl/interface/ftdi: auto-select SWD from converters' configs

tcl/interface/ftdi: auto-select SWD from converters' configs

When you source a JTAG-SWD converter config, any other transport
doesn't make any sense, so just autoselect it right there.

Change-Id: I6c098740905a0d4007473fc19cc07e11cbcc9369
Suggested-by: Хайруллин Эльдар <eldar.khayrullin@mail.ru>
Signed-off-by: Paul Fertser <fercerpav@gmail.com>
Reviewed-on: http://openocd.zylin.com/2248
Tested-by: jenkins
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>
Reviewed-by: Eldar Khayrullin <eldar.khayrullin@mail.ru>