ice: use pci_iov_vf_id() to get VF ID

The ice_sriov_set_msix_vec_count() obtains the VF device ID in a strange
way by iterating over the possible VF IDs and calling
pci_iov_virtfn_devfn to calculate the device and function combos and
compare them to the pdev->devfn.

This is unnecessary. The pci_iov_vf_id() helper already exists which does
the reverse calculation of pci_iov_virtfn_devfn(), which is much simpler
and avoids the loop construction. Use this instead.

Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>
Reviewed-by: Przemek Kitszel <przemyslaw.kitszel@intel.com>
Tested-by: Rafal Romanowski <rafal.romanowski@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>