ixgbe: add default setup_link for x550em_a MAC type

Add default setting for mac->ops.setup_link on x550em_a MAC types.
This fixes a link issue on KR parts.

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>