J-Link: Initialize .transports to suppress warning.

jtag_interface.transports field was left uninitialized, which triggers warning message
on program startup.

Although hardware natively supports SWD interface, no software support
currently present, so the value choosen to be <jtag_only>.

Change-Id: I2da41790b1850950af416cec4362d5b7bf927b2b
Signed-off-by: Alexander Osipenko <sipych@gmail.com>
Reviewed-on: http://openocd.zylin.com/670
Tested-by: jenkins
Reviewed-by: Xiaofan <xiaofanc@gmail.com>
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>

src/jtag/drivers/jlink.c

@@ -1298,6 +1298,7 @@ static const struct command_registration jlink_command_handlers[] = {
 struct jtag_interface jlink_interface = {
 	.name = "jlink",
 	.commands = jlink_command_handlers,
+	.transports = jtag_only,
 
 	.execute_queue = jlink_execute_queue,
 	.speed = jlink_speed,