e1000e: remove call to do_div and sign mismatch warning

Fixes a warning that was reported by Yanjiang Jin
<yanjiang.jin@windriver.com> by implementing the solution suggested by
Alexander Duyck <alexander.h.duyck@redhat.com>.

Signed-off-by: Yanir Lubetkin <yanirx.lubetkin@intel.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>