wireshark: use type-specific constructors for ProtoFields

This is apparently the more commonly used API. The generic constructor broke
compatibility between Wireshark releases 1.6 and 1.10.
diff --git a/wireshark_gen/field_info.py b/wireshark_gen/field_info.py
index 09011c7..2e9231f 100644
--- a/wireshark_gen/field_info.py
+++ b/wireshark_gen/field_info.py
@@ -27,29 +27,29 @@
 
 # Map from LOXI types to Wireshark types
 oftype_to_wireshark_type = {
-    "char": "INT8",
-    "uint8_t": "UINT8",
-    "uint16_t": "UINT16",
-    "uint32_t": "UINT32",
-    "uint64_t": "UINT64",
-    "of_mac_addr_t": "ETHER",
-    "of_ipv4_t": "IPv4",
-    "of_ipv6_t": "IPv6",
-    "of_port_name_t": "STRINGZ",
-    "of_table_name_t": "STRINGZ",
-    "of_desc_str_t": "STRINGZ",
-    "of_serial_num_t": "STRINGZ",
-    "of_octets_t": "BYTES",
-    "of_port_no_t": "UINT32",
-    "of_port_desc_t": "STRINGZ",
-    "of_bsn_vport_t": "BYTES",
-    "of_bsn_vport_q_in_q_t": "BYTES",
-    "of_fm_cmd_t": "UINT16",
-    "of_wc_bmap_t": "UINT64",
-    "of_match_bmap_t": "UINT64",
-    "of_match_t": "BYTES",
-    "of_oxm_t": "BYTES",
-    "of_meter_features_t": "BYTES",
+    "char": "int8",
+    "uint8_t": "uint8",
+    "uint16_t": "uint16",
+    "uint32_t": "uint32",
+    "uint64_t": "uint64",
+    "of_mac_addr_t": "ether",
+    "of_ipv4_t": "ipv4",
+    "of_ipv6_t": "ipv6",
+    "of_port_name_t": "stringz",
+    "of_table_name_t": "stringz",
+    "of_desc_str_t": "stringz",
+    "of_serial_num_t": "stringz",
+    "of_octets_t": "bytes",
+    "of_port_no_t": "uint32",
+    "of_port_desc_t": "stringz",
+    "of_bsn_vport_t": "bytes",
+    "of_bsn_vport_q_in_q_t": "bytes",
+    "of_fm_cmd_t": "uint16",
+    "of_wc_bmap_t": "uint64",
+    "of_match_bmap_t": "uint64",
+    "of_match_t": "bytes",
+    "of_oxm_t": "bytes",
+    "of_meter_features_t": "bytes",
 }
 
 # Map from LOXI type to Wireshark base