Fix bug, TableFeatures encode error

[shun159]

ofp_v4_encode:do(TF).
** exception error: no function clause matching ofp_v4_encode:table_feature_prop_field_id({nxm_1,31}) (src/ofp_v4_encode.erl, line 1038)
     in function  ofp_v4_encode:'-table_feature_prop_write_setfield/1-lc$^0/1-0-'/1 (src/ofp_v4_encode.erl, line 975)
     in call from ofp_v4_encode:'-table_feature_prop_write_setfield/1-lc$^0/1-0-'/1 (src/ofp_v4_encode.erl, line 975)
     in call from ofp_v4_encode:table_feature_prop_write_setfield/1 (src/ofp_v4_encode.erl, line 975)
     in call from ofp_v4_encode:'-table_features/1-lc$^0/1-0-'/1 (src/ofp_v4_encode.erl, line 850)
     in call from ofp_v4_encode:'-table_features/1-lc$^0/1-0-'/1 (src/ofp_v4_encode.erl, line 851)
     in call from ofp_v4_encode:table_features/1 (src/ofp_v4_encode.erl, line 850)
     in call from ofp_v4_encode:'-table_features_reply/1-lc$^0/1-0-'/1 (src/ofp_v4_encode.erl, line 839)