platform: cznic: turris-omnia-mcu: Fix error check in omnia_mcu_register_trng()

The gpiod_to_irq() function never returns zero.  It returns negative
error codes or a positive IRQ number.  Update the checking to check
for negatives.

Fixes: 41bb142a ("platform: cznic: turris-omnia-mcu: Add support for MCU provided TRNG")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Reviewed-by: Marek Behún <kabel@kernel.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>

drivers/platform/cznic/turris-omnia-mcu-trng.c

@@ -70,8 +70,8 @@ int omnia_mcu_register_trng(struct omnia_mcu *mcu)
 
 	irq_idx = omnia_int_to_gpio_idx[__bf_shf(OMNIA_INT_TRNG)];
 	irq = gpiod_to_irq(gpio_device_get_desc(mcu->gc.gpiodev, irq_idx));
-	if (!irq)
-		return dev_err_probe(dev, -ENXIO, "Cannot get TRNG IRQ\n");
+	if (irq < 0)
+		return dev_err_probe(dev, irq, "Cannot get TRNG IRQ\n");
 
 	/*
 	 * If someone else cleared the TRNG interrupt but did not read the