bebob: maintain clock control for Yamaha device

Yamaha Go series use selector function block command in 'AV/C Audio Subunit
Specification' to switch its clock source between 'Internal' and 'S/PDIF'.
Additionally detecting digital input is done by vendor specific AV/C command.

But just after switching from S/PDIF to Internal, get command returns
IN TRANSITION status. This transition continue around for 1500msec.