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: 43f01da0f2 ("MIPS/OCTEON/ata: Convert pata_octeon_cf.c to use device tree.")
Signed-off-by: Sergey Shtylyov <s.shtylyov@omp.ru>
Link: https://lore.kernel.org/r/3a241167-f84d-1d25-5b9b-be910afbe666@omp.ru
Signed-off-by: Jens Axboe <axboe@kernel.dk>

drivers/ata/pata_octeon_cf.c

@@ -898,10 +898,11 @@ static int octeon_cf_probe(struct platform_device *pdev)
 					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);
 		}