e1000e: check_polarity function pointers not set for 8257x

The function pointers for 8257x devices are not set.  This is not really a
problem now because there is nothing in the driver that references the
pointers for this particular MAC-family (the appropriate functions are
called directly), but the pointers should be set in case they are used in
the future.

Signed-off-by: Bruce Allan <bruce.w.allan@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>