[PKT_SCHED]: Fix datalen check in tcf_simp_init().

datalen is unsigned so it can never be less than zero,
but that's ok because the attribute passed to nla_len()
has been validated and therefore a negative return
value is impossible.

Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>