staging: wfx: wfx_init_common() returns NULL on error

Smatch complains:

    bus_spi.c:228 wfx_spi_probe() warn: 'bus->core' could be an error pointer
    bus_sdio.c:221 wfx_sdio_probe() warn: 'bus->core' could be an error pointer

bus->core contains the result of wfx_init_common(). With this patch,
wfx_init_common() returns a valid pointer or NULL.
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Jérôme Pouiller <jerome.pouiller@silabs.com>
Link: https://lore.kernel.org/r/20201009171307.864608-5-Jerome.Pouiller@silabs.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/wfx/main.c

@@ -309,7 +309,7 @@ struct wfx_dev *wfx_init_common(struct device *dev,
 	wdev->pdata.gpio_wakeup = devm_gpiod_get_optional(dev, "wakeup",
 							  GPIOD_OUT_LOW);
 	if (IS_ERR(wdev->pdata.gpio_wakeup))
-		return ERR_CAST(wdev->pdata.gpio_wakeup);
+		return NULL;
 	if (wdev->pdata.gpio_wakeup)
 		gpiod_set_consumer_name(wdev->pdata.gpio_wakeup, "wfx wakeup");