net: dsa: b53: Fix for failure when irq is not defined in dt

Fixes the issues with non BCM58XX chips in the b53 driver
failing, when the irq is not specified in the device tree.

Removed the check for BCM58XX in b53_srab_prepare_irq(),
so the 'port->irq' will be set to '-EXIO' if the irq is not
specified in the device tree.

Fixes: 16994374 ("net: dsa: b53: Make SRAB driver manage port interrupts")
Fixes: b2ddc48a ("net: dsa: b53: Do not fail when IRQ are not initialized")
Signed-off-by: Arun Parameswaran <arun.parameswaran@broadcom.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>