Fix infinite loop in selection between two SyncE clocks

[mlichvar]

This patch set should fix a problem when two connected SyncE clocks have external inputs and they are stuck in an infinite loop where:

• clock A selects clock B as its input
• clock A changes its transmitted QL to DNU
• clock B sees the change in QL of A and rebuilds its input priority, disconnects the external input, and changes its QL to DNU
• clock A sees the change in QL of B, rebuilds its input priority and selects its external input
• clock B locks to its external input again and changes its QL

[kubalewski]

LGTM