Bluetooth: btrtl: fix incorrect skb allocation failure check

BugLink: https://bugs.launchpad.net/bugs/1904221

Currently the check for a failed bt_skb_alloc allocation is incorrectly
checking using IS_ERR and this can lead to a null pointer dereference. Fix
this by checking for a null pointer return using the !skb idiom.

Addresses-Coverity: ("Dereference null return")
Fixes: 1996d9c ("Bluetooth: btrtl: Ask 8821C to drop old firmware")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>
(cherry picked from commit f5e8e21 linux-next)
Signed-off-by: Kai-Heng Feng <kai.heng.feng@canonical.com>
Signed-off-by: Timo Aaltonen <timo.aaltonen@canonical.com>

drivers/bluetooth/btrtl.c

@@ -578,7 +578,7 @@ struct btrtl_device_info *btrtl_initialize(struct hci_dev *hdev,
 		cmd[1] = opcode >> 8;
 
 		skb = bt_skb_alloc(sizeof(cmd), GFP_KERNEL);
-		if (IS_ERR(skb))
+		if (!skb)
 			goto out_free;
 
 		skb_put_data(skb, cmd, sizeof(cmd));