qdisc: basic classifier - remove unnecessary initialization

err is set once, then first code resets it.
  err = tcf_exts_validate(...)

Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>
Acked-by: Jamal Hadi Salim <hadi@mojatatu.com>
Signed-off-by: David S. Miller <davem@davemloft.net>