parport: fix parport_toggling_time regression

If parport_toggling_time is called before the adapter speed has been
configured then the call fails. Probably not the best fix, but does at least
enable parport_toggling_time to be used again.

This regression was added in commit 740b9e25

Change-Id: I90300916d6bda5ef053c557e5ac136c4f002bdd1
Signed-off-by: Spencer Oliver <spen@spen-soft.co.uk>
Reviewed-on: http://openocd.zylin.com/1309
Tested-by: jenkins