Bluetooth: Fix passing NULL to PTR_ERR

Passing NULL to PTR_ERR will result in 0 (success), also since the likes of
bt_skb_sendmsg does never return NULL it is safe to replace the instances of
IS_ERR_OR_NULL with IS_ERR when checking its return.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Tested-by: Tedd Ho-Jeong An <tedd.an@intel.com>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>