iavf: Fix failure to exit out from last all-multicast mode

The driver could only quit allmulti when allmulti and promisc modes are
turn on at the same time. If promisc had been off there was no way to turn
off allmulti mode.
The patch corrects this behavior. Switching allmulti does not depends on
promisc state mode anymore

Fixes: f42a5c74

 ("i40e: Add allmulti support for the VF")
Signed-off-by: Piotr Marczak <piotr.marczak@intel.com>
Tested-by: Tony Brelinski <tony.brelinski@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>