igb: clear VF_PROMISC bits instead of setting all other bits

This change corrects an issue in which we were setting all flag bits except
for promisc instead of clearing the promisc bits due to the incorrect use
of an |= instead of an &=.

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