vswitchd/bridge: Fix setting default prefix fields.

vswitchd/bridge: Fix setting default prefix fields.

The check for the need of default values was in the wrong place,
causing no prefix tracking to be used when database had no
configuration for a flow table.  Missing configuration means that
defaults should be used.

To limit clutter on the log, we now log the prefix tracking
configuration when it is explicitly set in the database.

Signed-off-by: Jarno Rajahalme <jrajahalme@nicira.com>
Acked-by: Ben Pfaff <blp@nicira.com>