igb: make local functions static and remove dead code

Based on Stephen Hemminger's original patch.
Make local functions static, and remove unused functions.

Reported-by: Stephen Hemminger <stephen@networkplumber.org>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>