ixgbe: cleanup sparse errors in new ixgbe_x550.c file

This patch cleans up prototypes that should have been defined
as static.

Signed-off-by: Don Skidmore <donald.c.skidmore@intel.com>
Tested-by: Phil Schmitt <phillip.j.schmitt@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>