rocker: Avoid unnecessary scheduling of work item

The work item function ofdpa_port_fdb_learn_work() does not do anything
when 'OFDPA_OP_FLAG_LEARNED' is not set in the work item's flags.

Therefore, do not allocate and do not schedule the work item when the
flag is not set.

Suggested-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>