The clock mode setting for this HDLC interface. This object is only
applicable to HDLC interfaces directly over a high speed serial
interface (HSSI).
At creation, if a value for this object is not specified, a default will
be chosen that is suitable for the type of lower interface on which this
interface is stacked. If the lower interface is not HSSI, then the
default value will be hdlcClockUnsupported(0); if the lower interface is
HSSI then the default value will be hdlcClockLine(2).