nfp: use callbacks for slow path ring related functions

To reduce the coupling of slow path ring implementations and their
callers, use callbacks instead.

Changes to Jakub's work:
* Also use callbacks for xmit functions

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