igb: make a few local functions static

Clean up a few sparse warnings, these following
functions can be made static:

drivers/net/ethernet/intel/igb/igb_main.c: warning: symbol
  'igb_add_mac_filter' was not declared. Should it be static?
drivers/net/ethernet/intel/igb/igb_main.c: warning: symbol
  'igb_del_mac_filter' was not declared. Should it be static?
drivers/net/ethernet/intel/igb/igb_main.c: warning: symbol
  'igb_set_vf_mac_filter' was not declared. Should it be static?

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>