ixgbe: cleanup make ixgbe_set_ethertype_anti_spoofing_X550 static

Correcting a mistake when I initial created this function.  I should
have made this static since it is only referenced where the function
pointer is assigned.

CC: <kernel-team@fb.com>
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>