i40e: refactor interrupt enable

The interrupt enable function was always making the caller add
the base_vector from the VSI struct which is already passed to
the function. Just collapse the math into the helper function.

Change-ID: I54ef33aa7ceebc3231c3cc48f7b39fd0c3ff5806
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>