ice: introduce ice_get_vf_by_dev() wrapper

The ice_get_vf_by_id() function is used to obtain a reference to a VF
structure based on its ID. The ice_sriov_set_msix_vec_count() function
needs to get a VF reference starting from the VF PCI device, and uses
pci_iov_vf_id() to get the VF ID. This pattern is currently uncommon in the
ice driver. However, the live migration module will introduce many more
such locations.

Add a helper wrapper ice_get_vf_by_dev() which takes the VF PCI device and
calls ice_get_vf_by_id() using pci_iov_vf_id() to get the VF ID.

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>