nfp: use TX ring pointer write back

Newer versions of the PCIe microcode support writing back the
position of the TX pointer back into host memory.  This speeds
up TX completions, because we avoid a read from device memory
(replacing PCIe read with DMA coherent read).

Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: Fei Qin <fei.qin@corigine.com>
Signed-off-by: Simon Horman <simon.horman@corigine.com>
Signed-off-by: David S. Miller <davem@davemloft.net>