iio: adc: at91-sama5d2_adc: remove dead code in at91_adc_probe()

From the comment of platform_get_irq(), it only returns non-zero IRQ
number and negative error number, other than zero.

Fix this by removing the if condition.

Signed-off-by: Cheng Ziqiu <chengziqiu@hust.edu.cn>
Reviewed-by: Dongliang Mu <dzm91@hust.edu.cn>
Reviewed-by: Claudiu Beznea <claudiu.beznea@microchip.com>
Link: https://lore.kernel.org/r/20230314070130.60581-1-chengziqiu@hust.edu.cn


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>