ixgbe: fix default mac->ops.setup_link for X550EM

X550EM_a/x did not have a default value for mac->ops.setup_link which
was causing link issues for backplane devices.

This patch sets mac->ops.setup_link to ixgbe_setup_mac_link_X540 for
X550EM_a/x which is also default for X550. This will result in
mac->ops.setup_link calling the link setup function for the respective
PHY type in case we do not need a special function to deal with it.

Reported-by: Ken Cox <jkc@redhat.com>
Signed-off-by: Emil Tantilov <emil.s.tantilov@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>