hla: add ability to change adapter speed (if supported)

hla: add ability to change adapter speed (if supported)

As a note we need to cache the requested speed setting, as the
hla interface may not be ready when the first adapter_khz is called.

Change-Id: I2fa6807d5f0bd3f0365cf178bd10a230c39415a7
Signed-off-by: Spencer Oliver <spen@spen-soft.co.uk>
Reviewed-on: http://openocd.zylin.com/2334
Tested-by: jenkins