pata_octeon_cf: avoid WARN_ON() in ata_host_activate()

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 <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>