After adding VLAN filter, the VLAN promiscuous mode is
disabled. But there's no chance to enable it.
So add the check after deleting VLAN filter. If there's
no VLAN filter left, enable the VLAN promiscuous mode.
Fixes: 9f0645cd147c ("net/i40e: fix VLAN filter")
Signed-off-by: Wenzhuo Lu <wenzhuo.lu@intel.com>