ofproto/xlate: Fix set field unwildcarding.

If the field does not exist, nothing is set.  However, we must
unwildcard the bits we used to make the decision, and we need not
unwildcard the field and it's prerequisities, if nothing is set.

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