jtag: simplify management of non-implemented handlers

jtag: simplify management of non-implemented handlers

There is just a single entry point for the jtag API .khz(),
.speed_div(), .power_dropout(), .srst_asserted().

Simplify the code by in-lining the default handler.
The overall code behaviour is not changed.
This change prevents modifying at run-time the content of
struct jtag_interface.

Inspired from change http://openocd.zylin.com/943
by Evan Hunter <ehunter@broadcom.com>

Change-Id: I09aeb76d614db57b1884ac7ee9f00c152cd77849
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: http://openocd.zylin.com/4892
Tested-by: jenkins
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>