ixgbe: Allow setting zero MAC address for VF

Currently, there is no logic that allows a VF's MAC address to be removed
from the RAR table.

Allow the user to specify a zero MAC address in order to clear the VF's
MAC address from the RAR table.  This functionality is also utilized by
libvirt when removing VFs.

Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>