drivers/jlink: fix SWD speed config, and set it before sending anything

drivers/jlink: fix SWD speed config, and set it before sending anything

During the initialisation a driver might need to communicate with the
target (e.g. sending jtag2swd sequence), so when doing so it should
honour the user-specified speed.

Change-Id: If84fea6057fda9edcf2c0a653edfbab2500e3cdd
[andrew.smirnov@gmail.com: fix khz/hz confusion]
Signed-off-by: Andrey Smirnov <andrew.smirnov@gmail.com>
Signed-off-by: Paul Fertser <fercerpav@gmail.com>
Reviewed-on: http://openocd.zylin.com/2224
Tested-by: jenkins
Reviewed-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>