jtag: set default "jtag_only" to uninitialized transports

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>