can: mcp251xfd: mcp251xfd_handle_rxif_one(): fix wrong NULL pointer check

If alloc_canfd_skb() returns NULL, 'cfg' is an uninitialized variable, so we
should check 'skb' rather than 'cfd' after calling alloc_canfd_skb(priv->ndev,
&cfd).

Fixes: 55e5b97f ("can: mcp25xxfd: add driver for Microchip MCP25xxFD SPI CAN")
Signed-off-by: Qinglang Miao <miaoqinglang@huawei.com>
Reviewed-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Link: https://lore.kernel.org/r/20210113073100.79552-1-miaoqinglang@huawei.comSigned-off-by: Marc Kleine-Budde <mkl@pengutronix.de>

drivers/net/can/spi/mcp251xfd/mcp251xfd-core.c

@@ -1491,7 +1491,7 @@ mcp251xfd_handle_rxif_one(struct mcp251xfd_priv *priv,
 	else
 		skb = alloc_can_skb(priv->ndev, (struct can_frame **)&cfd);
 
-	if (!cfd) {
+	if (!skb) {
 		stats->rx_dropped++;
 		return 0;
 	}