net: use core MTU range checking in WAN drivers

- set min/max_mtu in all hdlc drivers, remove hdlc_change_mtu
- sent max_mtu in lec driver, remove lec_change_mtu
- set min/max_mtu in x25_asy driver

CC: netdev@vger.kernel.org
CC: Krzysztof Halasa <khc@pm.waw.pl>
CC: Krzysztof Halasa <khalasa@piap.pl>
CC: Jan "Yenya" Kasprzak <kas@fi.muni.cz>
CC: Francois Romieu <romieu@fr.zoreil.com>
CC: Kevin Curtis <kevin.curtis@farsite.co.uk>
CC: Zhao Qiang <qiang.zhao@nxp.com>
Signed-off-by: Jarod Wilson <jarod@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>