igc: Handle already enabled taprio offload for basetime 0

Since commit e17090eb ("igc: allow BaseTime 0 enrollment for Qbv")
it is possible to enable taprio offload with a basetime of 0.
However, the check if taprio offload is already enabled (and thus -EALREADY
should be returned for igc_save_qbv_schedule) still relied on
adapter->base_time > 0.

This can be reproduced as follows:

    # TAPRIO offload (flags == 0x2) and base-time = 0
    sudo tc qdisc replace dev enp1s0 parent root handle 100 stab overhead 24 taprio \
	    num_tc 1 \
	    map 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 \
	    queues 1@0 \
	    base-time 0 \
	    sched-entry S 01 300000 \
	    flags 0x2

    # The second call should fail with "Error: Device failed to setup taprio offload."
    # But that only happens if base-time was != 0
    sudo tc qdisc replace dev enp1s0 parent root handle 100 stab overhead 24 taprio \
	    num_tc 1 \
	    map 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 \
	    queues 1@0 \
	    base-time 0 \
	    sched-entry S 01 300000 \
	    flags 0x2

Fixes: e17090eb ("igc: allow BaseTime 0 enrollment for Qbv")
Signed-off-by: Florian Kauer <florian.kauer@linutronix.de>
Reviewed-by: Kurt Kanzenbach <kurt@linutronix.de>
Tested-by: Naama Meir <naamax.meir@linux.intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>

drivers/net/ethernet/intel/igc/igc_main.c

@@ -6134,7 +6134,7 @@ static int igc_save_qbv_schedule(struct igc_adapter *adapter,
 	if (qopt->base_time < 0)
 		return -ERANGE;
 
-	if (igc_is_device_id_i225(hw) && adapter->base_time)
+	if (igc_is_device_id_i225(hw) && adapter->taprio_offload_enable)
 		return -EALREADY;
 
 	if (!validate_schedule(adapter, qopt))