ofproto: Use NULL "desc" values to indicate defaults.

ofproto: Use NULL "desc" values to indicate defaults.

This saves a small amount of memory.  It should also save a small amount of
time for reconfiguration starting with an upcoming commit where it becomes
possible to change the dp_desc away from the default (because it will not
be necessary to compare the current value against the default).

Signed-off-by: Ben Pfaff <blp@nicira.com>