igb: remove disable_av variable from mac_info struct

The disable_av variable is never used by the driver and provides no value as
it is likely a leftover debugging variable.  I have removed it and replaced
the one spot that checked for it with a check for a valid address.

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>