Bluetooth: Use IS_ERR_OR_NULL for checking bt_debugfs

Make sure to use IS_ERR_OR_NULL for checking the existing of the root
debugfs dentry bt_debugfs.

Signed-off-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>

diff --cc net/bluetooth/l2cap_core.c
Simple merge

diff --cc net/bluetooth/rfcomm/core.c
Simple merge

diff --cc net/bluetooth/rfcomm/sock.c
Simple merge

diff --cc net/bluetooth/sco.c
Simple merge