wan: convert drivers to netdev_tx_t

drivers/net/wan/cosa.c

drivers/net/wan/cycx_x25.c

drivers/net/wan/dlci.c

drivers/net/wan/dscc4.c

drivers/net/wan/farsync.c

drivers/net/wan/hd64570.c

drivers/net/wan/hd64572.c

drivers/net/wan/hdlc.c

drivers/net/wan/hdlc_fr.c

drivers/net/wan/hdlc_raw_eth.c

drivers/net/wan/hdlc_x25.c

drivers/net/wan/hostess_sv11.c

drivers/net/wan/lapbether.c

drivers/net/wan/lmc/lmc_main.c

drivers/net/wan/sbni.c

drivers/net/wan/sdla.c

drivers/net/wan/sealevel.c

drivers/net/wan/wanxl.c

drivers/net/wan/x25_asy.c

drivers/net/wan/z85230.c