dpif: Don't clear argument in dp_enumerate_types().

All but one of the callers of dp_enumerate_types() hands it an sset that is
known to be empty.  The one exception is the implementation of the
ofproto-provider 'enumerate_types' function in ofproto-dpif.  That function
isn't supposed to clear the existing contents of the sset passed in.
Therefore, this commit changes dp_enumerate_types() to not clear the sset
before it adds new members.

The ofproto-provider comment wasn't clear about the desired behavior so
this commit also clarifies that.

Reported-by: Zoltán Balogh <zoltan.balogh@ericsson.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>