e1000e: cleanup code duplication

The removed code block is duplicated in e1000e_write_itr() so use that
instead.

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>