bgpd: properly set peer->last_update

bgpd: properly set peer->last_update

Instead of checking whether the post-write number of updates sent was
greater than the pre-write number of updates sent, it was comparing post
to zero. In effect this meant every time we wrote a packet it was
counted as an update for route advertisement timer purposes.

Signed-off-by: Quentin Young <qlyoung@cumulusnetworks.com>