can: mcp251x: add error check when wq alloc failed

add error check when workqueue alloc failed, and remove redundant code
to make it clear.

Fixes: e0000163 ("can: Driver for the Microchip MCP251x SPI CAN controllers")
Signed-off-by: Weitao Hou <houweitaoo@gmail.com>
Acked-by: Willem de Bruijn <willemb@google.com>
Tested-by: Sean Nyekjaer <sean@geanix.com>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>