ixgbe: clean up ixgbe_get_settings ethtool function

This patch cleans up the method used for determining the link speed of
devices. The old method re-wrote some logic already existing in a mac.ops
function which should be used instead. The result is much simpler to
understand and removes a strange double-check of logic, as well as reducing
code redundancy.

Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>
Tested-by: Phil Schmitt <phillip.j.schmitt@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>