ixgbe: fix register access during ethtool loopback test

This patch cleans up the logic in ixgbe_setup_loopback_test() to only access
registers applicable to the MAC type. AUTOC is only valid on MACs older than
X540. MACC is used for X540.

In addition it removes a read of AUTOC and uses the stored value to force the
link up.

Signed-off-by: Emil Tantilov <emil.s.tantilov@intel.com>
Tested-by: Phil Schmitt <phillip.j.schmitt@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>