nfp: reuse ring helpers on .ndo_open() path

Ring allocation helpers encapsulate all ring allocation and
initialization steps nicely.  Reuse them on .ndo_open() path.

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