e1000e: Cleanup return values in ethtool

Changing occurrences of returning 0 and 1 from bool functions to false and
true, respectively

Signed-off-by: Dave Ertman <davidx.m.ertman@intel.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>