openflow: Value 4 of enum ofp15_group_mod_command is reserved.

The spec has been clarified to note that element 4 of
enum ofp_group_mod_command is reserved. This patch reflects
that change.

ONF-JIRA: EXT-350
Signed-off-by: Simon Horman <simon.horman@netronome.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>