e1000e: do not ignore variables which get set a value

Static analysis with cppcheck has shown a few instances of a variable which
is assigned a value that is never used.  A number of these are the return
status of various driver function calls which should be passed back to the
caller of the current function.

Signed-off-by: Bruce Allan <bruce.w.allan@intel.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>