misc: ss.c: fix logical error in main function

misc: ss.c: fix logical error in main function

In the line if (!dump_tcpdiag) { there was a logical error
in checking the descriptor, which the static analyzer complained
about (this action is always false)

fixed by replacing !dump_tcpdiag with !dump_fp

Reported-by: SVACE static analyzer
Signed-off-by: Anton Moryakov <ant.v.moryakov@gmail.com>
Acked-by: Stephen Hemminger <stephen@networkplumber.org>
Signed-off-by: David Ahern <dsahern@kernel.org>