Do not try to detach if the interface does not support IPv6.
Do not try to detach if the interface does not support IPv6.

Tested by:	hselasky
PR:		usb/182820
Approved by:	re (glebius)

jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase