ice: use pci_irq_vector helper function

Currently, driver gets interrupt number directly from ice_pf::msix_entries
array. Use helper function dedicated to do just that.

While at it use a variable to store interrupt number in
ice_free_irq_msix_misc instead of calling the helper function twice.

Reviewed-by: Michal Swiatkowski <michal.swiatkowski@linux.intel.com>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Tested-by: Pucha Himasekhar Reddy <himasekharx.reddy.pucha@intel.com> (A Contingent worker at Intel)
Signed-off-by: Piotr Raczynski <piotr.raczynski@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>