net: dsa: b53: Do not fail when IRQ are not initialized

When the Device Tree is not providing the per-port interrupts, do not fail
during b53_srab_irq_enable() but instead bail out gracefully. The SRAB driver
is used on the BCM5301X (Northstar) platforms which do not yet have the SRAB
interrupts wired up.

Fixes: 16994374 ("net: dsa: b53: Make SRAB driver manage port interrupts")
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>