jtag: set default "jtag_only" to uninitialized transports

For legacy support, drivers that do not define a list of
transports get identified as jtag_only.

Cleanup this old crust and initialize properly the transports
field in the jtag_interface for all the drivers.

Change-Id: I9c86064e5d05bd0212bc18f4424414e615e617fe
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: http://openocd.zylin.com/4893
Tested-by: jenkins
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>

src/jtag/minidummy/minidummy.c

@@ -28,6 +28,7 @@ struct jtag_interface minidummy_interface = {
 	.name = "minidummy",
 	.execute_queue = NULL,
 	.speed = NULL,
+	.transports = jtag_only,
 	.commands = NULL,
 	.init = NULL,
 	.quit = NULL,