Bluetooth: Fix MITM protection requirement preservation

If an existing connection has a MITM protection requirement (the first
bit of the auth_type) then that requirement should not be cleared by new
sockets that reuse the ACL but don't have that requirement.

Signed-off-by: Johan Hedberg <johan.hedberg@nokia.com>
Acked-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: Gustavo F. Padovan <padovan@profusion.mobi>