ice: Only disable VLAN pruning for the VF when all VLANs are removed

Currently if the VF adds a VLAN, VLAN pruning will be enabled for that VSI.
Also, when a VLAN gets deleted it will disable VLAN pruning even if other
VLAN(s) exists for the VF. Fix this by only disabling VLAN pruning on the
VF VSI when removing the last VF (i.e. vf->num_vlan == 0).

Signed-off-by: Brett Creeley <brett.creeley@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>