ftdi: fix adapter_init rclk fallback

adapter_init expects jtag_get_speed (via ftdi_khz) to return a valid
fallback speed if the adapter does not support rclk. The call was failing
and so was the rest of the adapter init.

The makes the new ftdi driver emulate the old ftdi driver.

Change-Id: Ic7fac7d201241eb181e98f1ba7111f159731f6e0
Signed-off-by: Spencer Oliver <spen@spen-soft.co.uk>
Reviewed-on: http://openocd.zylin.com/877
Tested-by: jenkins