Fix some trivial clang/gcc warnings in bsnmptc.c

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