igb: rework igb_set_multi so that vfs are properly updated

Currently if there are no multicast addresses programmed into the PF then
the VFs cannot have their multicast filters reset.  This change makes it so
the code path that updates vf multicast is always called along with the pf
updates.

Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>