net/sfc: rely on native thread safety in flow mechanism

net/sfc: rely on native thread safety in flow mechanism

The changeset that introduced common flow API thread safety
in fact introduced double locking to this particular PMD as
RTE flow API implementation in the PMD has been thread-safe
since the day zero. State this by setting the corresponding
device flag to skip locking imposed by generic RTE flow API.

Fixes: 80d1a9aff7f6 ("ethdev: make flow API thread safe")

Signed-off-by: Ivan Malov <ivan.malov@oktetlabs.ru>
Acked-by: Andrew Rybchenko <andrew.rybchenko@oktetlabs.ru>