net: cadence: macb: Enable software IRQ coalescing by default

This NIC doesn't have hardware IRQ coalescing. Under high load,
interrupts can adversely affect performance. To mitigate this, enable
software IRQ coalescing by default. On my system this increases receive
throughput with iperf3 from 853 MBit/sec to 934 MBit/s, decreases
interrupts from 69489/sec to 2016/sec, and decreases CPU utilization
from 27% (4x Cortex-A53) to 14%. Latency is not affected (as far as I
can tell).

Signed-off-by: Sean Anderson <[e-mail]>
Reviewed-by: Simon Horman <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

drivers/net/ethernet/cadence/macb_main.c

···
 		dev->ethtool_ops = &macb_ethtool_ops;
 	}
 
+	netdev_sw_irq_coalesce_default_on(dev);
+
 	dev->priv_flags |= IFF_LIVE_ADDR_CHANGE;
 
 	/* Set features */