nfp: don't add ring size to index calculations

Adding ring size to index calculation is pointless, since index
will be masked with ring size - 1.

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