Bluetooth: host: Fix SMP local keys check when starting encryption

Fix SMP check of existing local keys when attempting to start security
with required security mode 1 level 4. The logic for checking the
conditions was wrong, leading to a situation where encryption would be
attempted to be started by the central instead of initiating a new
pairing procedure. This would fail when the connection was encrypted and
the connection would be disconnected.

Signed-off-by: Joakim Andersson <joakim.andersson@nordicsemi.no>