ice: Remove vlan_ena from vsi structure

vlan_ena was introduced to track whether VLAN filters are enabled on
the device, but
1) checking for num_vlan > 1 already gives us this information, and is
currently used in this way throughout the code
2) the logic for vlan_ena is broken when multiple VLANs are active

Just remove vlan_ena and use num_vlan instead.

Signed-off-by: Nick Nunley <nicholas.d.nunley@intel.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>