The majority of the e1000e code checks most function return values using a
test like 'if (ret_val)' or 'if (!ret_val)' but there are a few instances
of 'if (ret_val == 0)'.  This patch converts the latter to the former for
consistency.
Signed-off-by: NBruce Allan <bruce.w.allan@intel.com>
Tested-by: NAaron Brown <aaron.f.brown@intel.com>
Signed-off-by: NJeff Kirsher <jeffrey.t.kirsher@intel.com>