i40e: expand comment

Explain why we cannot remove this code, even though it works differently
than any of our other interrupt cause handling code.

Change-ID: Ie66203bd037a466066036611c31d44f759ec5176
Signed-off-by: Mitch Williams <mitch.a.williams@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>