Bluetooth: SMP: Use JustWorks if none side requires MITM

From Core Spec Vol 3 Part H 2.3.5.1:
"If both devices have not set the MITM option in the Authentication
Requirements Flags, then the IO capabilities shall be ignored and the Just
Works association model shall be used."

Change-Id: I450f8ab5661382b787fe9742937d47df62fb6cfa
Signed-off-by: Szymon Janc <ext.szymon.janc@tieto.com>