jtag/drivers cmsis-dap: fix speed setting on Atmel EDBG

jtag/drivers cmsis-dap: fix speed setting on Atmel EDBG

Without the change Atmel EDBG uses default clock freq about 168 kHz
instead adapter_khz configured before interface init.
Changing adapter speed after init works as expected.

Testing shows the EDBG firmware resets speed to default during DAP_SWJ_Sequence.
Tested with fw versions 03.1F.01AE and 02.09.0169

This change repeats the DAP_SWJ_Clock command after sending a SWJ sequence.

Change-Id: Ic70457c5df635f47cad5e70b0dc83a083ea1b3a3
Reported-by: Ladislav Laska <laska@kam.mff.cuni.cz>
Signed-off-by: Tomas Vanek <vanekt@fbl.cz>
Reviewed-on: http://openocd.zylin.com/3944
Tested-by: jenkins
Reviewed-by: Freddie Chopin <freddie.chopin@gmail.com>