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

The macb ethernet driver (Raspberry Pi 5) delivers interrupts only to the first core, quickly saturating it at higher packet rates.

Introducing software interrupt coalescing dramatically alleviates this limitation; the oneliner fix is upstream at d57f7b45945ac0517ff8ea50655f00db6e8d637c.

Please backport this fix to 6.6 -stable to bring this benefit to more Raspberry Pis; it applies cleanly on this branch.

Many thanks, Daniel