nfp: rename netdev/port to vNIC

vNIC is a PCIe-side abstraction NFP firmwares supported by this
driver use.  It was initially meant to represent a device port
and therefore a netdev but today should be thought of as a way
of grouping descriptor rings and associated state.  Advanced apps
will have vNICs without netdevs and ports without a vNIC (using
representors instead).

Make sure code refers to vNICs as vNICs and not ports or netdevs.
No functional changes.

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