[BeBoB/Mixer] make 'EnhancedMixer' path work
The bebob_mixer class gives 'EnhancedMixer' path but actually this doen't work because it has no implementation. For BeBoB/MAudio devices, ffado-dbus-server needs to execute 'Processing function block command' via this interface.
'EnhancedMixer' command is BridgeCo's extension of 'Processing function block' command. The latter is in 'AV/C Audio Subunit Specification 1.0 (1394TA Document
1999008)'. Unfortunately there is no way to distinguish them by
plug information.
This commit add an limitation that 'EnhancedMixer' path can't use the other types of 'Processing function block' command than 'single type'. But actually FFADO developer have neter confirmed the device which use the other types of command.
git-svn-id: svn+ssh://ffado.org/ffado/trunk@2444 2be59082-3212-0410-8809-b0798e1608f0