i40e/i40evf: refactor IRQ enable function

This change moves a multi-line register setting into a function
which simplifies reading the flow of the enable function.

This also fixes a bug where the enable function was enabling
the interrupt twice while trying to update the two interrupt
throttle rate thresholds for Rx and Tx.

Change-ID: Ie308f9d0d48540204590cb9d7a5a7b1196f959bb
Signed-off-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>