Revert "net: caif: replace BUG_ON with recovery code"

This reverts commit c5dea815834c7d2e9fc633785455bc428b7a1956.

Because of recent interactions with developers from @umn.edu, all
commits from them have been recently re-reviewed to ensure if they were
correct or not.

Upon review, this commit was found to be incorrect for the reasons
below, so it must be reverted.  It will be fixed up "correctly" in a
later kernel change.

The original change here was pointless as dev can never be NULL in this
function so the claim in the changelog that this "fixes" anything is
incorrect (also the developer forgot about panic_on_warn).  A follow-up
change will resolve this issue properly.

Cc: Aditya Pakki <pakki001@umn.edu>
Cc: David S. Miller <davem@davemloft.net>
Link: https://lore.kernel.org/r/20210503115736.2104747-19-gregkh@linuxfoundation.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/net/caif/caif_serial.c

@@ -269,9 +269,7 @@ static netdev_tx_t caif_xmit(struct sk_buff *skb, struct net_device *dev)
 {
 	struct ser_device *ser;
 
-	if (WARN_ON(!dev))
-		return -EINVAL;
-
+	BUG_ON(dev == NULL);
 	ser = netdev_priv(dev);
 
 	/* Send flow off once, on high water mark */