ixgbevf: fix null pointer dereference due to filter being set for VLAN 0

This change corrects an issue that resulted in a null pointer dereference
for the addition of VLAN 0 without any VLANs being registered.  Also this
code removes some unnecessary checks for defines and the unnecessary setting
of VLAN flags since that is now handled within the kernel via the
vlan_features.

Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
Tested-by: Emil Tantilov <emil.s.tantilov@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>