ixgbevf: Drop netdev_registered value since that is already stored in netdev

There is no need to keep a separate netdev_registered value since that is
already stored in the netdev itself.

Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
Acked-by: Greg Rose <gregory.v.rose@intel.com>
Tested-by: Sibai Li <sibai.li@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>