nfp: loosen relation between rings and IRQs vectors

Upcoming XDP support will break the assumption that one can iterate
over IRQ vectors to get to all the rings easily.  Use nn->.x_ring
arrays directly.

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