mlxsw: spectrum: Allow fetch-and-clear of flow counters

For the report_delta-like interface like a previous patch has added for
collection of NH group statistics, it's easiest to read the counter and
have the HW clear it right away. Thus, change mlxsw_sp_flow_counter_get()
to take a bool indicating whether this should be done.

Signed-off-by: Petr Machata <petrm@nvidia.com>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Link: https://lore.kernel.org/r/6a096ede8ee92d5041e3832242c3bbc137198aba.1709901020.git.petrm@nvidia.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>