Bug 5864: link biblio => serials subscription is wrong

Bug 5864: link biblio => serials subscription is wrong

The GetSubscription function can be called with 3 parameters : title,
issn, biblionumber.
In biblio view (both opac & staff interface), the link to jump from the
biblio to the subscription(s) was done using title.

The result was that for a title like "Science", all subscriptions
containing "science" in the title were linked.
That's too much and that's false.

This commit fixes this problem by calling GetSubscription with
biblionumber only where applicable.

Signed-off-by: Magnus Enger <magnus@enger.priv.no>
Signed-off-by: Chris Cormack <chrisc@catalyst.net.nz>