net: caif: remove BUG_ON(dev == NULL) in caif_xmit

The condition of dev == NULL is impossible in caif_xmit(), hence it is
for the removal.

Explanation:
The static caif_xmit() is only called upon via a function pointer
`ndo_start_xmit` defined in include/linux/netdevice.h:
```
struct net_device_ops {
    ...
    netdev_tx_t     (*ndo_start_xmit)(struct sk_buff *skb, struct net_device *dev);
    ...
}
```

The exhausive list of call points are:
```
drivers/net/ethernet/qualcomm/rmnet/rmnet_map_command.c
    dev->netdev_ops->ndo_start_xmit(skb, dev);
    ^                                    ^

drivers/infiniband/ulp/opa_vnic/opa_vnic_netdev.c
    struct opa_vnic_adapter *adapter = opa_vnic_priv(netdev);
			     ^                       ^
    return adapter->rn_ops->ndo_start_xmit(skb, netdev); // adapter would crash first
	   ^                                    ^

drivers/usb/gadget/function/f_ncm.c
    ncm->netdev->netdev_ops->ndo_start_xmit(NULL, ncm->netdev);
	      ^                                   ^

include/linux/netdevice.h
static inline netdev_tx_t __netdev_start_xmit(...
{
    return ops->ndo_start_xmit(skb, dev);
				    ^
}

    const struct net_device_ops *ops = dev->netdev_ops;
				       ^
    rc = __netdev_start_xmit(ops, skb, dev, more);
				       ^
```

In each of the enumerated scenarios, it is impossible for the NULL-valued dev to
reach the caif_xmit() without crashing the kernel earlier, therefore `BUG_ON(dev ==
NULL)` is rather useless, hence the removal.

Cc: David S. Miller <davem@davemloft.net>
Signed-off-by: Du Cheng <ducheng2@gmail.com>
Link: https://lore.kernel.org/r/20210503115736.2104747-20-gregkh@linuxfoundation.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/net/caif/caif_serial.c

@@ -269,7 +269,6 @@ static netdev_tx_t caif_xmit(struct sk_buff *skb, struct net_device *dev)
 {
 	struct ser_device *ser;
 
-	BUG_ON(dev == NULL);
 	ser = netdev_priv(dev);
 
 	/* Send flow off once, on high water mark */