rme: clock source selection and management on the RME devices is a bit trickier than...

rme: clock source selection and management on the RME devices is a bit trickier than a simple list implies.  Down the track it might be possible to use the standard clock source list functionality in some way, but for now fake it out with a dummy clock source to prevent warnings about no clock sources being active (and so on).

git-svn-id: svn+ssh://ffado.org/ffado/trunk@2035 2be59082-3212-0410-8809-b0798e1608f0