i40e/i40evf: Add txring_txq function to match fm10k and ixgbe

i40e/i40evf: Add txring_txq function to match fm10k and ixgbe

This patch adds a txring_txq function which allows us to convert a
i40e_ring/i40evf_ring to a netdev_tx_queue structure.  This way we
can avoid having to make a multi-line function call for all the spots
that need access to this.

Change-ID: Ic063b71d8b92ea406d2c32e798c8e2b02809d65b
Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>