ofproto-dpif: Move definition of "struct subfacet" earlier.

ofproto-dpif: Move definition of "struct subfacet" earlier.

The next commit will add an instance of struct subfacet as a member of
struct facet, so struct subfacet must be declared first.

This commit moves around code without otherwise changing it.

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