Bluetooth: Add smp_ltk_sec_level() helper function

There are several places that need to determine the security level that
an LTK can provide. This patch adds a convenience function for this to
help make the code more readable.

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

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

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

diff --cc net/bluetooth/smp.h
Simple merge