wifi: mt76: testmode: add nla_policy for MT76_TM_ATTR_TX_LENGTH

It seems that the nla_policy in mt76_tm_policy is missed for attribute
MT76_TM_ATTR_TX_LENGTH. This patch adds the correct description to make
sure the

  u32 val = nla_get_u32(tb[MT76_TM_ATTR_TX_LENGTH]);

in function mt76_testmode_cmd() is safe and will not result in
out-of-attribute read.

Fixes: f0efa862 ("mt76: add API for testmode support")
Signed-off-by: Lin Ma <linma@zju.edu.cn>
Signed-off-by: Felix Fietkau <nbd@nbd.name>