nfp: create separate define for max number of vectors

We are currently using define for max TX rings to allocate IRQ
vectors.  It's OK since the max number of rings for TX and RX
are currently the same, but lets make the code nicer by taking
max of the two.

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