igb: Cleanups to fix missing break in switch statements

This patch fixes WARNING:MISSING_BREAK found with checkpatch check.

Signed-off-by: Carolyn Wyborny <carolyn.wyborny@intel.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>