Fix some trivial clang/gcc warnings in bsnmptc.c

• By definition, enum snmp_tc can't be false (the implied starting sequence index for the enum is 0). Don't test for it being < 0.
• Staticize struct snmp_text_conv to mute a -Wmissing-variable-declarations warning from clang.
• Remove set but unused variable, ptr, in parse_bridge_id(..) and parse_bport_id(..) to mute warning from gcc 4.9+.
• Mark value and string unused in snmp_inetaddr2oct(..) and parse_inetaddr(..) as they're just stub functions.

MFC after: 1 week
Sponsored by: EMC / Isilon Storage Division