mtd: nand: atmel: fix of_irq_get() error check

of_irq_get() may return 0 as well as negative error number on failure,
while the driver only checks for the negative values. The driver would
then call devm_request_irq() for IRQ0  in its probe method and never get
a valid interrupt.

Check for 'nc->irq <= 0' instead and return -ENXIO from the driver's probe
if of_irq_get() returned 0.

Fixes: f88fc122 ("mtd: nand: Cleanup/rework the atmel_nand driver")
Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Acked-by: Wenyou Yang <Wenyou.yang@microchip.com>
Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>