e1000e: PHY type cleanups in e1000e_check_downshift()

Remove the case for 82577 because it does not support the ability to check
for downshift.  Add case for e1000_phy_bm which can do this.

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>