ixgbe: Use correct FC setup function for x550em_a

Somehow the wrong fc_setup function was used for x550em_a, so
correct that. Also set setup_link to NULL as its value is
determined later, just like it is with X550EM_x.

Signed-off-by: Mark Rustad <mark.d.rustad@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>