nfp: flower: fix used time of merge flow statistics

commit 5b186cd6 upstream.

Prior to this change the correct value for the used counter is calculated
but not stored nor, therefore, propagated to user-space. In use-cases such
as OVS use-case at least this results in active flows being removed from
the hardware datapath. Which results in both unnecessary flow tear-down
and setup, and packet processing on the host.

This patch addresses the problem by saving the calculated used value
which allows the value to propagate to user-space.

Found by inspection.

Fixes: aa6ce2ea ("nfp: flower: support stats update for merge flows")
Signed-off-by: Heinrich Kuhn <heinrich.kuhn@netronome.com>
Signed-off-by: Simon Horman <simon.horman@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>