ui: Always enable NIC independent fields

Previously the VLANID and Bridge fields in the NIC Details dialog were
disabled soemtimes.
This is now fixed by keeping them enabled all the time.

Change-Id: Ia1c07bb095d48303a6e30405d7ed467aa712f5b5
Signed-off-by: Fabian Deutsch <fabiand@fedoraproject.org>