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()...