ixgbe: make __ixgbe_setup_tc static

This function is only used in ixgbe_main.c
Resolves a "missing prototype" warning when building the driver with W=1

Reported-by: Phil Schmitt <phillip.j.schmitt@intel.com>
Signed-off-by: Emil Tantilov <emil.s.tantilov@intel.com>
Acked-by: John Fastabend <john.r.fastabend@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>