can: mcp251x: mcp251x_can_probe(): add missing unregister_candev() in error path

In mcp251x_can_probe(), if mcp251x_gpio_setup() fails, it forgets to
unregister the CAN device.

Fix this by unregistering can device in mcp251x_can_probe().

Fixes: 2d52dabb

 ("can: mcp251x: add GPIO support")
Signed-off-by: Dongliang Mu <dzm91@hust.edu.cn>
Link: https://lore.kernel.org/all/20221024090256.717236-1-dzm91@hust.edu.cn
[mkl: adjust label]
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>