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.com
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>