HDLC_MAX_MTU
dev->mtu = HDLC_MAX_MTU;
dev->max_mtu = HDLC_MAX_MTU;
dev->mtu = HDLC_MAX_MTU;
dev->max_mtu = HDLC_MAX_MTU;
#define HDLC_MAX_MRU (HDLC_MAX_MTU + 10 + 14 + 4) /* for ETH+VLAN over FR */