bgpd: Convert bgp_packet_mpattr_prefix to use an enum for safi's

bgpd: Convert bgp_packet_mpattr_prefix to use an enum for safi's

The function bgp_packet_mpattr_prefix was using an if statement
to encode packets to the peer.  Change it to a switch and make
it handle all the cases and fail appropriately when something
has gone wrong.  Hopefully in the future when a new afi/safi
is added we can catch it by compilation breaking instead of
weird runtime errors

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