e1000e: Program the correct register for ITR when using MSI-X.

When configuring interrupt throttling on 82574 in MSI-X mode, we need to
be programming the EITR registers instead of the ITR register.

-rc2: Renamed e1000_write_itr() to e1000e_write_itr(), fixed whitespace
      issues, and removed unnecessary !! operation.
-rc3: Reduced the scope of the loop variable in e1000e_write_itr().

Signed-off-by: Matthew Vick <matthew.vick@intel.com>
Acked-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>