nfp: flower: fix stats id allocation

As flower rules are added, they are given a stats ID based on the number
of rules that can be supported in firmware. Only after the initial
allocation of all available IDs does the driver begin to reuse those that
have been released.

The initial allocation of IDs was modified to account for multiple memory
units on the offloaded device. However, this introduced a bug whereby the
counter that controls the IDs could be decremented before the ID was
assigned (where it is further decremented). This means that the stats ID
could be assigned as -1/0xfffffff which is out of range.

Fix this by only decrementing the main counter after the current ID has
been assigned.

Fixes: 467322e2627f ("nfp: flower: support multiple memory units for filter offloads")
Signed-off-by: John Hurley <john.hurley@netronome.com>
Reviewed-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>