ixgbe: Add X550EM_x dual-speed SFP+ support

This patch adds X550EM_x SFP+ dual-speed support. 82599 fiber link
code was moved from ixgbe_82599.c to ixgbe_common.c for use by
X550EM. SFP MAC link code is added to x550EM.

Signed-off-by: Mark Rustad <mark.d.rustad@intel.com>
Tested-by: Phil Schmitt <phillip.j.schmitt@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>