igb: fix vlan filtering in promisc mode when not in VT mode

This patch fixes a VT mode check to make sure VLAN filters are disabled when
in promisc mode and VT is not enabled.

The problem with the previous check was that:
E1000_MRQC_ENABLE_VMDQ is defined as 0x00000003

but when not in VT mode:
mrqc |= E1000_MRQC_ENABLE_RSS_4Q (0x00000002)

So the above check will trigger regardless if VT mode is being used or not.

Signed-off-by: Emil Tantilov <emil.s.tantilov@intel.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>