Allow "tcl_port" to be run after config to simply display the port.

To be consistent with gdb_port and telnet_port, allow tcl_port
to be run to display the configured port number, while still
not allowing the user to change it after configuration.

Change-Id: Ibe6aedb3bd447f2985e42dd8246358481a047dd6
Signed-off-by: Robert P. J. Day <rpjday@crashcourse.ca>
Reviewed-on: http://openocd.zylin.com/1857
Tested-by: jenkins
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>