Fix authenticator PMKSA cache expiration timer setup

Fix authenticator PMKSA cache expiration timer setup

The PMKSA cache expiration timer was not actually ever initialized since
the only place for registering the timeout was in the timeout handler.
Fix this by initializing the timer whenever a new PMKSA cache entry is
added to the beginning of the list (i.e., when it was the first entry or
expires before the entry that was previously going to expire first).
[Bug 393]

Signed-hostap: Jouni Malinen <j@w1.fi>