ixgbe: fix setup_fc for x550em

mac->ops.setup_fc can be null for backplanes which can cause the driver
to crash on load.

Reported-by: Patrick McLean <patrickm@gaikai.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>