stm8 target: make adapter speed settings work

stm8 target: make adapter speed settings work

Previously the adapter speed settings were hard-coded to
connect with low speed then switch over to high speed
regardless what was mentioned in the cfg files. Now the
stm8 target intercept adapter speed settings and configure
the stm8 control registers accordingly.

Change-Id: I7419514e5214e4b43b9d51253cf5b7f04a233533
Signed-off-by: Ake Rehnman <ake.rehnman@gmail.com>
Reviewed-on: http://openocd.zylin.com/5548
Tested-by: jenkins
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>