sysfsgpio: enable only the transport specific gpio

sysfsgpio: enable only the transport specific gpio

If the configuration file specifies both SWD and JTAG gpios, the
current code request all of them. In case of overlap a warning is
generated when the same gpio is released for the second time.

Require and release only the gpio needed by the specified
transport.

Change-Id: I41a0970980ceeb559afa98ab34cfe93dffed2e1c
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: http://openocd.zylin.com/5556
Tested-by: jenkins