Bluetooth: Update the security level when link is encrypted

If the pending security level is greater than the current security
level and the link is now encrypted, we should update the link
security level.

This is only useful for LE links, when the only event generated
when SMP is sucessful in the Encrypt Change event.

Signed-off-by: Vinicius Costa Gomes <vinicius.gomes@openbossa.org>
Signed-off-by: Gustavo F. Padovan <padovan@profusion.mobi>