iavf: Add helper function to go from pci_dev to adapter

Add helper function to go from pci_dev to adapter to make work simple -
to go from a pci_dev to the adapter structure and make netdev assignment
instead of having to go to the net_device then the adapter.

Signed-off-by: Brett Creeley <brett.creeley@intel.com>
Signed-off-by: Karen Sornek <karen.sornek@intel.com>
Tested-by: Konrad Jankowski <konrad0.jankowski@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>