cw1200: make cw1200_spi_irq_unsubscribe() void

Fix the following coccicheck warning:

drivers/net/wireless/st/cw1200/cw1200_spi.c:273:5-8: Unneeded variable:
"ret". Return "0" on line 279
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Jason Yan <yanaijie@huawei.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20200410090910.27132-1-yanaijie@huawei.com

drivers/net/wireless/st/cw1200/cw1200_spi.c

@@ -268,15 +268,11 @@ static int cw1200_spi_irq_subscribe(struct hwbus_priv *self)
 	return ret;
 }
 
-static int cw1200_spi_irq_unsubscribe(struct hwbus_priv *self)
+static void cw1200_spi_irq_unsubscribe(struct hwbus_priv *self)
 {
-	int ret = 0;
-
 	pr_debug("SW IRQ unsubscribe\n");
 	disable_irq_wake(self->func->irq);
 	free_irq(self->func->irq, self);
-
-	return ret;
 }
 
 static int cw1200_spi_off(const struct cw1200_platform_data_spi *pdata)