net/sched: taprio: replace tc_taprio_qopt_offload :: enable with a "cmd" enum

Inspired from struct flow_cls_offload :: cmd, in order for taprio to be
able to report statistics (which is future work), it seems that we need
to drill one step further with the ndo_setup_tc(TC_SETUP_QDISC_TAPRIO)
multiplexing, and pass the command as part of the common portion of the
muxed structure.

Since we already have an "enable" variable in tc_taprio_qopt_offload,
refactor all drivers to check for "cmd" instead of "enable", and reject
every other command except "replace" and "destroy" - to be future proof.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Horatiu Vultur <horatiu.vultur@microchip.com> # for lan966x
Acked-by: Kurt Kanzenbach <kurt@linutronix.de> # hellcreek
Reviewed-by: Muhammad Husaini Zulkifli <muhammad.husaini.zulkifli@intel.com>
Reviewed-by: Gerhard Engleder <gerhard@engleder-embedded.com>
Signed-off-by: David S. Miller <davem@davemloft.net>