carp: Don't set a link level address if vhid == -1

Link level address for carp is dervied from vhid.
Until vhid is set, carp is useless, so don't give it a link level address
until a vhid is set.

This fixes recent test case breakage where carp was fixed to actually
print the ethernet address set by default. Note that neither carp nor
the test case itself was actually broken as the error is the common
ATF net code assuming that a cloned interface's link level address is
unique upon creation.

(roy)