can: c_can: fix null-ptr-deref on ioctl()

The pdev maybe not a platform device, e.g. c_can_pci device, in this
case, calling to_platform_device() would not make sense. Also, per the
comment in drivers/net/can/c_can/c_can_ethtool.c, @bus_info should
match dev_name() string, so I am replacing this with dev_name() to fix
this issue.

[    1.458583] BUG: unable to handle page fault for address: 0000000100000000
[    1.460921] RIP: 0010:strnlen+0x1a/0x30
[    1.466336]  ? c_can_get_drvinfo+0x65/0xb0 [c_can]
[    1.466597]  ethtool_get_drvinfo+0xae/0x360
[    1.466826]  dev_ethtool+0x10f8/0x2970
[    1.467880]  sock_ioctl+0xef/0x300

Fixes: 2722ac98 ("can: c_can: add ethtool support")
Link: https://lore.kernel.org/r/20210906233704.1162666-1-ztong0001@gmail.com
Cc: stable@vger.kernel.org # 5.14+
Signed-off-by: Tong Zhang <ztong0001@gmail.com>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>

drivers/net/can/c_can/c_can_ethtool.c

@@ -15,10 +15,8 @@ static void c_can_get_drvinfo(struct net_device *netdev,
 			      struct ethtool_drvinfo *info)
 {
 	struct c_can_priv *priv = netdev_priv(netdev);
-	struct platform_device *pdev = to_platform_device(priv->device);
-
 	strscpy(info->driver, "c_can", sizeof(info->driver));
-	strscpy(info->bus_info, pdev->name, sizeof(info->bus_info));
+	strscpy(info->bus_info, dev_name(priv->device), sizeof(info->bus_info));
 }
 
 static void c_can_get_ringparam(struct net_device *netdev,