pata_octeon_cf: avoid WARN_ON() in ata_host_activate()

Iff platform_get_irq() fails (or returns IRQ0) and thus the polling mode
has to be used, ata_host_activate() hits the WARN_ON() due to 'irq_handler'
parameter being non-NULL if the polling mode is selected. Let's only set
the pointer to the driver's IRQ handler if platform_get_irq() returns a
valid IRQ # -- this should avoid the unnecessary WARN_ON()...

Fixes: 43f01da0f279 ("MIPS/OCTEON/ata: Convert pata_octeon_cf.c to use device tree.")
Signed-off-by: Sergey Shtylyov <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Jens Axboe <[e-mail]>

drivers/ata/pata_octeon_cf.c

···
 					return -EINVAL;
 				}
 
-				irq_handler = octeon_cf_interrupt;
 				i = platform_get_irq(dma_dev, 0);
-				if (i > 0)
+				if (i > 0) {
 					irq = i;
+					irq_handler = octeon_cf_interrupt;
+				}
 			}
 			of_node_put(dma_node);
 		}