ice: use irq_num var in ice_vsi_req_irq_msix

Someone went through the effort of making this a variable so let's use
it instead of recalculating it again.

Signed-off-by: Alan Brady <alan.brady@intel.com>
Signed-off-by: Anirudh Venkataramanan <anirudh.venkataramanan@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>