jtag_interface: .speed can be NULL  when not needed

adapter_init (core.c) won't check speed configuration
of the selected interface if it's not needed (.speed = NULL).

When it's not needed, we can now omit adapter_khz in
init scripts and we don't have to implement dummy handlers
for speed_div and khz functions.

It also removes calls to adapter_khz in interface configuration
files when not used anymore.

Change-Id: I6eb1894385503fede542a368f297cec6565eed44
Signed-off-by: Franck Jullien <franck.jullien@gmail.com>
Reviewed-on: http://openocd.zylin.com/1131
Tested-by: jenkins
Reviewed-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>