Bluetooth: Refactor bt_conn_security

Factor out role dependent code to helper. This allows to get rid of
'done' label without decresing code readibility. Allows to cleanly
build with CONFIG_BLUETOOTH_PERIPHERAL and CONFIG_BLUETOOTH_SMP
defined.

Change-Id: I33606955ae8b1c75385e2eee89620761d59f0108
Signed-off-by: Szymon Janc <ext.szymon.janc@tieto.com>