nfp: replace num_irqs with max_r_vecs

num_irqs is not used anywhere, replace it with max_r_vecs which holds
number of allocated RX/TX vectors and is going to be useful soon.

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