tcl/target/stm32f4: fix: reduce adapter speed before reset

tcl/target/stm32f4: fix: reduce adapter speed before reset

The reset-init hook for this target speeds up the CPU clock and JTAG adapter
speed. When the target is reset running with high adapter speed, a series of
warnings "DAP transaction stalled (WAIT) - slowing down" will be generated
since the adapter speed is not reduced to fit the slower CPU speed.
Fix: reduction of the adapter speed before a reset is performed.

Change-Id: Iabfc8e3f70311e0e71c8eed09b8a37fcbed9c58d
Signed-off-by: Alexander Kurz <akurz@blala.de>
Reviewed-on: http://openocd.zylin.com/3365
Tested-by: jenkins
Reviewed-by: Uwe Bonnes <bon@elektron.ikp.physik.tu-darmstadt.de>
Reviewed-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>