can: mcp251x: mcp251x_setup(): remove unused parameter "struct mcp251x_priv *priv"

The 2nd parameter of mcp251x_setup() "struct mcp251x_priv *priv" is not
used, so remove it.

Suggested-by: Oleksij Rempel <o.rempel@pengutronix.de>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>