bgpd: Clean up peer status checking for a received nlri

bgpd: Clean up peer status checking for a received nlri

In bgp_update_receive the first thing we do is establish
that the peer->status is Established.  We then do a bunch
of work and call bgp_nlri_parse where we break out for
each address family.  Each AFI is then checking for
being peer->status is Established again.  There is no
point in checking this again.

Signed-off-by: Donald Sharp <sharpd@cumulusnetworks.com>