Bluetooth: Don't use incomplete LE LTKs to secure link

If LE link gets terminated during the pairing process, it
may leave SMP with half of a Long Term Key. This fix ensures
that this partial information is not used for future links.

Change-Id: Ib834fc81d7b4b558cc3d242522acef80dd1d8827
Signed-off-by: Brian Gix <bgix@codeaurora.org>