环境描述：
openwrt 18.06 (Linux-4.9.120)
问题描述：
iPhone XR usb tethering出现内核崩溃
root@GL-MIFI:/# [ 253.130282] ------------[ cut here ]------------
[ 253.133546] WARNING: CPU: 0 PID: 7145 at net/sched/sch_generic.c:316 dev_watchdog+0x174/0x260
[ 253.142008] NETDEV WATCHDOG: eth2 (ipheth): transmit queue 0 timed out
[ 253.148478] Modules linked in: ath9k ath9k_common rt2800usb rt2800lib pppoe ppp_async option ath9k_hw ath usb_wwan sierra_net sierra rt2x00usb rt2x00lib rndis_host qmi_wwan pppox ppp_mppe ppp_generic nf_nat_pptp nf_conntrack_pptp nf_conntrack_ipv6 mac80211 iptable_nat ipt_REJECT ipt_MASQUERADE huawei_cdc_ncm cp210x cfg80211 cdc_ncm cdc_ether xt_time xt_tcpudp xt_tcpmss xt_statistic xt_state xt_recent xt_nat xt_multiport xt_mark xt_mac xt_limit xt_length xt_hl xt_helper xt_hashlimit xt_ecn xt_dscp xt_conntrack xt_connmark xt_connlimit xt_connbytes xt_comment xt_TCPMSS xt_REDIRECT xt_L