i40e: move enabling icr0 into i40e_update_enable_itr

If we don't have MSI-X enabled, we handle interrupts on all icr0. This
is a special case, so let's move the conditional into
i40e_update_enable_itr() in order to make i40e_napi_poll easier to
read about.

Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>