Bluetooth: SMP: Fix dropping subsequent Security Requests

It is possible that slave sends subsequent Security Request while
link is already encrypted. One example is that current LTK is
unauthenticated and slave wants to increase security with MITM
protection.

Change-Id: I5f079e6140a5912443f770ba2c7cabeffcecdf2b
Signed-off-by: Szymon Janc <ext.szymon.janc@tieto.com>