jtag/drivers/ftdi: Use adapter_khz value for initial clock speed (was default 1kHz)

jtag/drivers/ftdi: Use adapter_khz value for initial clock speed (was default 1kHz)

nrf51822 doesn't like a 1kHz initial clock rate, puts the DAP into a bad state.

Mailing list discussion thread: http://sourceforge.net/p/openocd/mailman/openocd-devel/thread/20140718081528.GA5554%40ex2.lan/#msg32621853

Change-Id: I71aa75505cc1b41ee80c7b9db415f6ac738c2916
Signed-off-by: Angus Gratton <gus@projectgus.com>
Reviewed-on: http://openocd.zylin.com/2223
Tested-by: jenkins
Reviewed-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>