Fix value of OFPG_MAX

This constant was introduced with the value 0xffffff00 in version 1.3.1
of the spec.  Fix our macro to use the correct value.

Motivated by FlowForwarding/LINC-Switch#191.
Fix typo in type name ofp_group_mod_failed
Merge pull request #50 from legoscia/fix-OFPG_MAX

Fix value of OFPG_MAX