e1000e: cleanup ethtool loopback setup code

Refactor the loopback setup code to first handle the only 10/100 PHY
supported by the driver if applicable and then handle the 1Gig PHYs in a
switch statement for PHY-specific setups.

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